摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of the Chemical Society Faraday Transactions Scientific Editor Dr. Peter J. Sarre Department of Chemistry University of Notting ham University Park Nottingham NG7 2RD, UK Faraday Editorial Board Prof. I. W. M. Smith (Birmingham) (Chairman) Prof. M. N. R. Ashfold (Bristol) Dr. B. E. Hayden (Southampton) Dr. D. C. Clary (Cambridge) Prof. A. R. Hillman (Leicester) Dr. L. R. Fisher (Bristol) Prof. J. Holzwarth (Berlin) Prof. H. M. Frey (Reading) Dr. P. J. Sarre (Nottingham) Dr. R. K. Thomas (Oxford) Editorial Manager and Secretary to Faraday Editorial Board Dr. Robert J. Parker The Royal Society of Chemistry Thomas Graham House Science Park Milton Road Cambridge CB4 4WF, UK Senior Assistant Editors: Mrs.S. Shah, Dr. R. A. Whitelock Assistant Editor: Mrs. C. J. Seeley Editorial Secretary: Mrs. J. E. Gibbs International Advisory Editorial Board R. S. Berry (Chicago) Y. Marcus (Jerusalem) A. M. Bradshaw (Berlin) B. J. Orr (North Ryde) A. Carrington (Southampton) R. H. Ottewill (Bristol) M. Che (Paris) R. Parsons (Southampton) M. S. Child (Oxford) S. L. Price (London) B. E. Conway (Ottawa) F. Rondelez (Paris) G. R. Fleming (Chicago) J. P. Simons (Oxford) R. Freeman (Cambridge) S. Stolte (Amsterdam) H. L. Friedman (Stony Brook) J. Troe (Gottingen) H. lnokuchi (Okazaki) J. Wolfe (Kensington, NSW) J. N. lsraelachvili (Santa Barbara) C. Zannoni (Bologna) M. L. Klein (Philadelphia) A. Zecchina (Turin) R. A. Marcus (Pasadena) C. Zhang (Dalian) Journal of the Chemical Society, Faraday Transactions (ISSN 0956-5000) is published twice monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Black- horse Road, Letchworth, Herts. SG6 lHN, UK. 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Dr. P. J. Sarre, Scientific Editor. Tel.: Nottingham (0602) 51 3465 (24 hours) E-Mail (JANET): PCZPSF@U K.AC.NOlT.VAX Fax: (0602) 51 3466 Telex: 37346 UNINOT G Dr. R. J. Parker, Editorial Manager. Tel. : Cambridge (0223) 420066 E-Mail (INTERNET): RSCl @RSC.ORG (For access from JANET use RSC1% RSC.ORG@UK.AC.NSF NET-R ELAY) Fax: (0223) 423623 or 420247 Telex: 81 8293 ROYAL G

ISSN:0956-5000    

DOI:10.1039/FT99490FX033

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0956-5000    

DOI:10.1039/FT99490BX035

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0956-5000 JCFTEV(9) 1197-1 363 (1994) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions Physical Chemistry & Chemical Physics CONTENTS 1197 Investigation into the kinetics and mechanism of the reaction of NO, with CH, and CH,O at 298 K between 0.6 and 8.5 Torr: Is there a chain decomposition mechanism in operation? P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross and R. P. Wayne 1205 Investigation into the kinetics and mechanism of the reaction of NO, with CH302 at 298 K and 2.5 Torr: A potential source of OH in the night-time troposphere? P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross and R. P. Wayne 1211 Molecular conformations and rotation barriers of 2-halogenoethanethiols and 2-halogenoethanols : An ab initio study G.Buemi . 1217 Thermophysical properties of liquid m-xylene at high pressures M. Taravillo, S. Castro, V. G. Baonza, M. Caceres and J. Nuiiez 1223 Heats of transport of aqueous tetraalkylammonium hydroxides and the electrophoretic effect D. G. Leaist and L. Hao 1227 Complexation and precipitation equilibria in the system Ni"-CrV'-H,O R. Castano, M. A. Olazabal, G. Borge and J. M. Madariaga 1233 Profiles of adsorption during the oxidation of small organic molecules: Oxidation of formic acid at polycrystalline Pt in acid solutions C. P. Wilde and M. Zhang 1241 Influence of polymer structure on the electrochemistry of phenothiazine dyes incorporated into Nafion films S. A. John and R. Ramaraj 1245 Integral equation theory for associating liquids: Highly asymmetric electrolytes J.Wang and A. D. J. Haymet 1251 Evanescent wave spectroscopy: Application to the study of the spatial distribution of charged groups on an adsorbed polyelectrolyte at the silica/water interface M. Trau, F. Grieser, T. W. Healy and L. R. White 1261 Comparison of the electrokinetic properties of the silica surface D. E. Dunstan 1265 Vibrational spectroscopic analysis of Group 6 metal hexacarbonyls in the solid state U. A. Jayasooriya 1271 Valence and core photoemission of the films formed electrochemically on nickel in sulfuric acid Y. Liang, P. M. A. Sherwood and D. K. Paul .'1279 Nickel incorporated into anodic porous alumina formed on an aluminium wire N. Ohji, N. Enomoto, 'I..Mizushima, N. Kakuta, Y.Morioka and A. Ueno 1285 Modification of the electronic structure of Pd by U films: Chemisorption of CO T. H. Gouder and C. A. Colmenares 1293 Spectroscopic characterization of magnesium vanadate catalysts. Part 2.-FTIR study of the surface properties of pure and mixed-phase powders G. Ramis, G. Busca and V. Lorenzelli 1301 l80tracer studies of CO oxidation with 0,on MOO,. Part 1.-Diffusion of "0atoms from active sites during the catalysis and the determination of the number of active sites Y. Iizuka 1307 tracer studies of CO oxidation with 0, on MOO,. Part 2.-Active sites for CO oxidation with 0, and for oxygen isotope exchange between CO, and MOO, Y. Iizuka, H. Tanigaki, M. Sanada, J. Tsunetoshi, N. Yamauchi and S. Arai 1313 Conformational and vibrational properties of a,w-dihalogenoalkane/urea inclusion compounds :A Raman scattering investigation S.P. Smart, A. El Baghdadi, F. Guillaume and K. D. M. Harris 1323 Conformational properties of monosubstituted cyclohexane guest molecules constrained within zeolitic host materials. A solid-state NMR investigation A. E. Aliev, K. D. M. Harris and R. C. Mordi 1329 Solid-state ion exchange in zeolites. Part 5.-NH,-Y-iron(11) chloride K. Lazar, G. Pal-BorMly, H. K. Beyer and H. G. Karge I 1335 Manganese-promoted rhodium/NaY zeolite catalysts. An IR spectroscopic study H. Treviiio, 2.C. Zhang, W. M. H. Sachtler, C.Dossi, R. Psaro and R. Ugo T. Beutel, H. Knozinger, 1345 Zinc-exchangedY zeolites studied with carbon monoxide and xenon as probes B.Boddenberg and A. Seidel 1351 Adsorption of benzene on the acidic and basic sites of KH fl zeolite studied by in situ spectroscopy T. Sun, D-z. Jiang and E-z. Min J-p. Shen, J. Ma, FARADAY COMMUNICATIONS 1355 Solubility of hydrogen and deuterium in Ti,Al M. Kimura, T. Tsuchiyama, S. Naito and M. Yamamoto ~~ ~ ~ 1357 Book reviews N. M. D. Brown; M. N. R. Asbfold D. B. Sellen; C.D. Flint; A. A. Herd; G. Duxbury; D. A. Dunmur; 1363 Corrigendum to Structural studies on paracyanogen and paraisocyanogen L. W. Jenneskens, J. W. G. Maby, E.J. Vliestra, S. J. Goede and F. Bickelhaupt Note: Where an asterisk appears against the name of one or more of the authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles. For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W 1V OBN, UK Tel: +44 (0)71-437 8656 Fax: +44 (0)71-287 9798 Telecom Gold 84: BUR210 Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note’ that copies are not available from the RSC at Thomas Graham House, Cambridge.

ISSN:0956-5000    

DOI:10.1039/FT99490FP085

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aas, N., 1015 Caceres Alonso, M., 553 Feliu, J. M., 609 Ikonnikov, I. A., 219 Machado, V. G., 865Afanasiev, P., 193 Calado, J. C. G., 649 Filimonov, I. N., 219, 227 Indovina, V., 207 Mackie, J. C., 541Aikawa, M., 911 Caldararu, H., 21 3 Flint, C. D., 1357 Inoue, Y., 797,815 Mackintosh, J. G., 1121 Aitken, C. G., 935 Calvente, J. J., 575 Fogden, A., 263 Ishiga, F., 979 Macpherson, A. N., 1065Akanuma, K., 1171 Calvo, E. J., 987 Fornts, V., 213 Ishigure, K., 93, 591 Madariaga, J. M., 1227 Akolekar, D. B., 1041 Camacho, J. J., 23 Fracheboud, J-M., 1197, Isoda, T., 869 Maeda, T., 899 Albery, W. J., 1115 Cameron, B. R., 935 1205 Ito, O., 571 Maestre, A., 575Aldaz,A., 609 Campa, M. C., 207 Franck, R., 667,675 Iwasaki, K., 121 Maginn, S.J., 1003Alfimov, M. V., 109 Campos, A., 339 Freeman, N. J., 751 Jacobs, W. P. J. H., 1191 Mahy, J. W. G., 327,1363Al-Ghefaili, K. M., 383, Canosa-Mas, C. E., 1197, Frety, R., 773 Jakubov, T., 783 Maity, D. K., 703 1047 1205 Frey, J. G., 17, 817 Jameel, A. T., 625 Makarova, M. A., 383,Ali, V., 579, 583 Capobianco, J. A., 755 Frostemark, F., 559 Janchen, J., 1033 1047Aliev, A. E., 1323 Caragheorgheopol, A., 213 Fujiwara, Y., 1183 Jayakumar, R., 161 Maksymiuk, K., 745Allegrini, P., 333 Carlile, C. J., 1149 Gandolfi, R., 1077 Jayasooriya, U. A., 1265 Malatesta, V., 333 Allen, N. S., 83 Carlsen, L., 941 Gans, P., 315 Jenneskens, L. W., 327, Malcolm, B. R., 493 Al Rawi, J. M. A., 845 Carvill, B. T., 233 Gao,Y., 803 1363 Mallon, D., 83 Amorim da Costa, A.M., Castaiio, R., 1227 Garcia, R., 339 Jennings, B. J., 55 Mandal, A. B., 161 689 Castro, S., 1217 Garcia-Paiieda, E., 575 Jiang, D-z., 1351 Marcheselli, L., 859 Amoskov, V. M., 889 Catalina, F., 83 Gautam, P., 697 Jiang, P-Y., 591 Marchetti, A., 859, 1089 Ando, M., 1011 Cavasino, F. P., 311 Geantet, C., 193 Jiang, P. Y., 93 Mariani, M., 423 Andrews, S. J., 1003 Chang, T-h., 1157 Gengembre, L., 895 Jobic, H., 1191 Martins, A., 143 Aragno, A., 787 Charlesworth, P., 1073 Gil, A. M., 1099 Johansson, L. B.-A., 305 Maruya, K-i., 91 1 Arai, S., 1307 Chen, J-S., 429, 717 Gil, F. P. S. C., 689 Johari, G. P., 883, 1143 Masetti, F., 333 Aramaki, K., 321 Chen, Y-H., 617 Gilchrist, J., 1149 John, S. A., 1241 Massucci, M., 445 Aravindakumar, C. T., 597 Cheng, A., 253 Gill, D.S., 579, 583 Joseph, E. M., 387 Matijevic, E., 167 Asai, Y., 797 Cheng, C. P., 1157 Gill, J. B., 315 Joshi, P. N., 387 Matsuda, J., 321 Ashfold, M. N. R., 1357 Cherqaoui, D., 97 Goede, S. J., 327, 1363 Kagawa, S., 349 Matsumura, Y., 1177 Avila, V., 69 Chesta, C. A., 69 Gomez, C. M., 339 Kakuta, N., 1279 May, I. P., 751 Baba,T., 187 Chevalier, S., 667, 675 Gonqalves da Silva, A. M., Kaler, E. W., 471 Mazzucato, U., 333 Badri, A., 1023 Chmiel, G., 1153 649 Kalugin, 0.N., 297 McGilvery, D., 1055 Bagatti, M., 1077 Cho,T., 103 Gouder, T. H., 1285 Karge, H. G., 1329 Mchedlov-Petrossyan, N. O.,Ball, M. C., 997 Christensen, P., 459 Gray, P. G., 369 Kato, R., 763 629 Ball, S. M., 523 Climent, M. A,, 609 Green, W. A., 83 Katsumura, Y., 93, 591 McNaughton, D., 1055 Baonza, V.G., 553, 1217 Coates, J. H., 739 Grein, F., 683 Kaur,T., 579 Medforth, C. J., 1073 Barbaux, Y., 895 Colmenares, C. A., 1285 Grieser, F., 1251 Kawashima, T., 127 Melrose, J. R., 1133 Barthomeuf, D., 667,675 Cordischi, D., 207 Griflith, W. P., 1105 Keil, M., 403 Merga, G., 597 Basini, L., 787 Corma,A., 213 Grimshaw, J., 75 Kemball, C., 659 Meunier, F., 369 Bassoli, M., 363 Cormier, G., 755 Grzybowska, B., 895 Kessel, D., 1073 Mezyk, S. P., 831 Battaglini, F., 987 Corradini, F., 859, 1089 Guelton, M., 895 Kida, I., 103 Min, E-z., 1351 Bauer, C., 517 Corrales, T., 83 Guillaume, F., 1313 Kiennemann, A., 501 Misono, M., 1183 Bell, A. J., 17, 817 Cosa, J. J., 69 Gulliya, K. S., 953 Kim, J-H., 377 Mitchell, P.J., 1133 Belton, P. S., 1099 Cottier, D., 1003 Hachey, M., 683 Kimura, M., 1355 Mittal, J. P., 597, 703, 711, Bendig, J., 287 Coudurier, G., 193 Haeberlein, M., 263 King, F., 203 825 Bengtsson, L. A., 559 Courcot, D., 895 Hall, D. I., 517 Kirschner, J., 403 Miyake, Y., 979 Benko, J., 855 Crawford, M. J., 817 Hall, G., 1 Kita, H., 803 Mizuno, N., 1183 Benniston, A. C., 953 Cullis, P. M., 727 Hallbrucker, A., 293 Klein, M. L., 253 Mizushima, T., 1279 Bensalem, A., 653 Curtis, J. M., 239 Halpern, A., 721 Kleshchevnikova, V. N., Moffat, J. B., 1177 BCrces, T., 41 1 Dang, N-T., 875 Hamnett, A., 459 629 Mohan, H., 597,703Bergeret, G., 773 Danil de Namor, A. F., 845 Hancock, G., 523 Knozinger, H., 1335 Monk, P. M. S., 1127 Beutel, T., 1335 Das, T.N., 963 Handa, H., 187 Kobayashi, A., 763 Mordi, R. C., 1323 Beyer, H. K., 1329 Dasannacharya, B. A., 1149 Hann,K., 733 Kobayashi, H., 763 Moriguichi, I., 349 Bickelhaupt, F., 327, 1363 Davey, R. J., 1003 Hao, L., 133, 1223 Kobayashi, T., 1011 Morikawa, A., 377 Biczok, L., 411 Davidson, K., 879 Harada, S., 869 Kondo, Y., 121 Morioka, Y., 1279 Biggs, P., 1197, 1205 Demeter, A., 41 1 Haraoka, T., 911 Kossanyi, J., 41 1 Morokuma, M., 377 Binet, C., 1023 Dempsey, P., 1003 Harland, P. W., 935 Kurrat, R., 587 Morrison, C. A., 755 Black, S. N., 1003 Demri, D., 501 Harper, R. J., 659 Kuwamoto, T., 121 Mount, A. R., 1115, 1121 Blackett, P. M., 845 Derrick, P. J., 239 Harriman, A,, 697,953 Laachir, A., 773 Muir, A. V. G., 459 Blandamer, M. J., 727 Dewing, J., 1047 Harris, K.D. M., 1313, Lajtar, L., 1153 Mukherjee, T., 711 Blower, C., 919,931 Diagne, C., 501 1323 Lambert, J-F., 667,675 Mukhopadhyay, R., 1149 Boddenberg, B., 1345 Dickinson, E., 173 Harrison, N. J., 55 Lamotte, J., 1029 Nagaishi, R., 93, 591 Boggis, S. A., 17 Doblhofer, K., 745 Haruta, M., 1011 Langan, J. R., 75 Nagaoka, H., 349 Borge, G., 1227 Domen, K., 911 Hashimoto, K., 1177 Lavalley, J-C., 1023, 1029 Naito, S., 899, 1355 Borisenko, V. N., 109 Dossi, C., 1335 Hashino, T., 899 Lavanchy, A., 783 Naito, T., 763 Boutonnet-Kizling, M., Doughty, A,, 541 Hattori, H., 803 Lizar, K., 1329 Navaratnam, S., 83 1023 Douglas, C. B., 471 Haymet, A. D. J., 1245 Lazzarini, E., 423 Neoh, K. G., 355 Bowker, M., 1015 Dunmur, D. A,, 1357 Heal, M. R., 523 Leaist, D.G., 133, 1223 Nerukh, D. A., 297 Bozon-Verduraz, F., 653 Dunstan, D. E., 1261 Healy, T. W., 1251 Lei,G-D., 233 Nicholson, D., 181 Bradley, C. D., 239 Duxbury, G., 1357 Heenan, R. K., 487 Lerner, B. A., 233 Nickel, U., 617 Bradshaw, A. M., 403 Dwyer, J., 383, 1047 Helmer, M., 31, 395 Leslie, M., 641 Ninomiya, J., 103 Braun, B. M., 849 Dyke, J. M., 17 Herein, D., 403 Li, J., 39 Nishihara, H., 321 Breysse, M., 193 Eastoe, J., 487 Herod, A. A., 1357 Li, P., 605 Nogami, T., 763 Briggs, B., 727 Easton, C. J., 739 Herzog, B., 403 Li, Y., 947 Nonaka, O., 121 Brocklehurst, B., 271 Ebitani, K., 377 Heyes, D. M., 1133 Liang, Y., 1271 Nuiiez, J., 1217 Brown, N. M. D., 1357 Egsgaard, H., 941 Higgins, S., 459 Lin, J., 355 Nuiiez Delgado, J., 553 Brown, R.G., 59 El-Atawy, S., 879 Hindermann, J-P., 501 Lincoln, S. F., 739 Nyholm, L., 149 Brown, S. E., 739 El Baghdadi, A., 1313 Hirst, D. M., 517 Lindblom, G., 305 Occhiuzzi, M., 207,905 Bruna, P. J., 683 Elisei, F., 279 Hiyane, I., 973 Liu,C-W., 39 Ohji, N., 1279 Brzezinski, B., 843, 1095 Elliot, A. J., 831, 837 Hoekstra, D., 727 Liu, X., 249 Ohtsu, K., 127 Buckley, A. M., 1003 Engberts, J. B. F. N., 727 Holmberg, B., 559 Loginov, A. Yu., 219,227 Okamura, A., 803 Buemi, G., 1211 Enomoto, N., 1279 Holz, M., 849 Lohse, U., 1033 Olazabal, M. A,, 1227 Burdisso, M., 1077 Eustaquio-Rinc6n, R., 113 Hoshino, H., 479 Longdon, P. J., 315 Olejnik, J., 1095 Busca, G., 1161,1293 Ewins, C., 969 Hosoi, K., 349 Lorenzelli, V., 1293 Oliveri, G., 363 Butt, M.D., 727 Fantola Lazzarini, A. L., Hutchings, G. J., 203 Lu, J-X., 39 Onishi, T., 91 1 Byatt-Smith, J. G., 493 423 Hutton, R. S., 345 Lunelli, B., 137 Ono,Y., 187 Cabaleiro, M. C., 845 Fausto, R., 689 Iizuka, Y., 1301, 1307 Ma, J., 1351 Oradd, G., 305 Caceres, M., 1217 Favaro, G., 279,333 Ikawa, S-i., 103 Mabuchi, M., 899 Ortica, F., 279 1 Oswal, S. L., 1083 Ota, K-i., 155 Otlejkina, E. G., 297 Otsuka, K., 451 Ottavi, G., 333 Ouellette, D. C., 837 Owari, T., 979 Ozutsumi, K., 127 Padley, M. B., 203 Pal, H., 711 Pal-Borbkly, G., 1329 Palleschi, A., 435 Rao, B. S. M., 597 Rastelli, A., 1077 Rehani, S. K., 583 Rettig, W., 59 Rey, F., 213 Rezende, M. C., 865 Rhodes, N. P., 809 Ricchiardi,G., 1161 Richter, R., 17 Robertson, E. G., 1055 Rocha, M., 143 Rochester, C.H., 203 Shen, J-p., 1351 Sheppard, N., 507,513 Sherwood, P. M. A., 1271 Shiao, J-C., 429 Shihara, Y., 549 Shiralkar, V. P., 387 Shishido, T., 803 Shizuka, H., 533 Siders, P., 973 Silva, C. J., 143 Silva, F., 143 Simkiss, K., 641 Terarnoto, M., 979 Teraoka, Y., 349 Thompson, K. M., 1105 Thompson, N. E., 1047 Timms, A. W., 83 Timney, J. A., 459 Togawa, T., 1171 Tomkinson, J., 1149 Tosi, G., 859, 1089 Touret, O., 773 Tournayan, L., 773 Trau, M., 1251 Wang, C. F., 605 Wang, J., 1245 Watanabe, H., 571 Waters, M., 727 Wayne, R. P., 1197,1205 Weckstrom, K., 733 Weingartner, H., 849 Weir, D. J., 751 Werner, H., 403 Whitaker, B. J., 1 White, L. R., 1251 Whitehead, M. A., 47 Paradisi, C., 137 Pardo, A., 23 Parsons, B. J., 83 Patel, S. G., 1083 Pathmanathan, K., 1143 Patrykiejew, A., 1153 Paul, D.K., 1271 Pavanaja, U. B., 825 Pedulli, G. F., 137 Peters, M. P. J., 1033 Pen& W., 605 Pepe,F., 905 Pereira, C. M., 143 Ptrez, J. M., 609 Perrichon, V., 773 Peter, L. M., 149 Petrov, N. Kh., 109 Pispisa, B., 435 Pivnenko, N. S., 297 Plane, J. M. C., 31,395 Plowman, R., 1003 Porcar, I., 339 Potter, C. A. S., 59 Poyato, J. M. L., 23 Prenosil, J. E., 587 Previtali, C. M., 69 Pringle, T. J., 1015 Priyadarsini, K. I., 963 Pryamitsyn, V. A., 889 Psaro, R., 1335 Rabold, A., 843 Ramaraj, R., 1241 Rama Rao, K. V. S., Ramis, G., 1293 Ramsden, J. J., 587 825 Rodes, A., 609 Roffa,S., 137 Rosenholm, J. B., 733 Rosmus, P., 517 Rosseinsky, D. R., 1127 Rossi, P. F., 363 Rout, J. E., 1003 Rudham, R., 809 Ryde,N., 167 Sacco, A., 849 Sachtler, W.M. H., 233, Saitoh, T., 479 Salmon, G. A., 75 Sam, D. S. H., 1161 Sanada, M., 1307 Sano,T., 869 Sapre, A. V., 825 Sarre, P. J., 517 Sato, K., 797 Saur, O., 1029 Sbriziolo, C., 311 Schedel-Niedrig, Th., 403 Schlogl, R., 403 Schnabel, W., 287 Scremin, M., 865 Seddon, B. J., 605 Seidel, A., 1345 Sellen, D. B., 1357 Shahid, G., 507,513 Shallcross, D. E., 1197, Sharma,A., 625 Shaw, N., 17,817 Sheil, M. M., 239 1335 1205 Singh, J., 579,583 Singh, R., 583 Smart, S. P., 1313 Smith, K. M., 1073 Smith, T. D., 919,931 Soares, V. A. M., 649 Sokdowski, S., 1153 Soria,V., 339 Spiro, M., 61 7, 1105 Stanley, D. R., 1003 Stewart, B., 969 Stoeckli, F., 783 Sun, L. M., 369 Sun,T., 1351 Suquet, H., 667,675 Surov, Y. N., 297 Suzuki, T., 549 Tabata, M., 1171 Tabrizchi, M., 17 Tagliazucchi, M., 859,1089 Takagi, T., 121 Takahashi, K., 155 Takasawa, A., 911 Tamaura, Y., 1171 Tamura, K-i., 533 Tanaka,I., 349 Tanigaki, H., 1307 Taravillo, M., 1217 Tassi, L., 859, 1089 Tateno, A., 763 Tatham, A,, 1099 Taylor, A., 1003 Taylor, M.G., 641 Teixeira-Dim, J. J. C., 689 Teo, W. K., 355 Trejo, A., 113 Treviiio, H., 1335 Truscott, T. G., 1065,1073 Tsuchiyama, T., 1355 Tsuji, H., 803 Tsuji, M., 1171 Tsunashima, S., 549 Tsunetoshi, J., 1307 Tung, C-H., 947 Turco Liveri, M. L., 311 Turco Liveri, V., 311 Turner, P. H., 1065 Udagawa, T., 763 Ueno, A., 1279 Ugo,R., 1335 Umemoto, H., 549 Unayama, S-i., 549 Upadhyaya, H. P., 825 Valat, P., 411 Valls, M. J., 609 van Hooff,J. H. C., van Santen, R. A., 1191 van Wolput, J. H. M. C., Vedrine, J. C., 193 Venanzi, M., 435 Villamagna, F., 47 Villemin, D., 97 Visscher, P.B., 1133 Vlietstra, E. J., 327, 1363 VollhrovA, O., 855 Vollmer, F., 59 Vyunnik, I. N., 297 Wales, D. J., 1061 1033 1033 Volt% J-C., 1161 Wikander, G., 305 Wilde, C. P., 1233 Williams, D. E., 345 Wilpert, A., 287 Wintgens, V., 411 Woermann, D., 875 Wohlers, M., 403 Wolthuizen, J. P., 1033 Wormald, C. J., 445 Xin, Q., 973 Yagci, Y., 287 Yamaji, M., 533 Yamamoto, M., 899, 1355 Yamanaka, I., 451 Yamasaki, M., 869 Yamauchi, N., 1307 Yanes, C., 575 Yang, Z-Q., 947 Yano,H., 869 Yasuda, H., 1183 Yeh, C-t., 1157 Yoshitake, H., 155 Yotsuyanagi, T., 93,479 Young, R. N., 271 Zanotto, S. P., 865 Zhang, M., 1233 Zhang, X., 605 Zhang, Z. C., 1335 Zholobenko, V. L., 233, Zhong, G. M., 369 Ziolek, M., 1029 Zubarev, V.E., 721 Zundel, G., 843,1095 1047 11 ~ ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Statistical Mechanics and Thermodynamics Group Cellular Automata and their Applications to Molecular Fluids To be held at the University of Manchester on 19 and 20 July 1994 Further information from Dr A. Masters, Department of Chemistry, University of Manchester, Manchester M13 9PL Division Autumn Meeting: Reactions and Mechanisms for Fine Chemicals To be held at the University of Glasgow on 6-9 September 1994 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Gas Kinetics Group 13th International Symposium on Gas Kinetics To be held at University College, Dublin on 11-15 September 1994 Further information from Dr H.Sidebottom, Department of Chemistry, University College, Dublin Electrochemistry Group with the SCI ELECTROCHEM 94 To be held in Edinburgh on 12-16 September 1994 Further information from Professor D. E. Williams, Department of Chemistry, University College London, 20 Gordon Street, London WClH OAJ Biophysical Chemistry Group with the Industrial Division Biotechnology Group Peptide + Water = Protein To be held at University College, London on 19 September 1994 Further information from Professor J. L. Finney, Department of Physics and Astronomy, University College London, Gower Street, London WClE 6BT ~ British Carbon Group Applications of Microporous Carbons To be held at the University of Leeds on 28 and 29 September 1994 Further information from Professor B.Rand, Department of Chemistry, The University, Leeds LS2 9JT Theoretical Chemistry Group with CCPl Electronic Structure: From Molecules to Enzymes To be held at University College London on 30 November 1994 Further information from Dr P. J. Knowles, School of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ Division Annual Congress: Lasers in Chemistry To be held at Heriot Watt University, Edinburgh on 10-13 April 1995 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London WlV OBN Division Joint Meeting with the Division de Chimie Physique de la Societe' Francaise de Chimie, Deutsche Bunsen Gesellschaft fur Physikalische Chemie and Associazione Italiana di Chimica Fisica Fast Elementary Processes in Molecular Systems To be held at the UniversitC De Lille, France on 16-30 June 1995 Further information from Dr C.Troyanowsky ,Division de Chimie Physique, Laboratoire de Chimie Physique, 11 rue Pierre et Marie Curie, 75005 Paris, France British Carbon Group Carbon '96 To be held at the University of Newcastle upon Tyne on 7-12 July 1996 Further information from Dr K. M. Thomas, Northern Carson Research Laboratories, The University, Newcastle upon Tyne NE1 7RU iii THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 98 Polymers at Surfaces and Interfaces University of Bristol, 12-14 September 1994 Organising Committee: Professor Sir Sam Edwards (Chairman) Dr R.Buscall Professor R. H. Ottewill Dr T. Cosgrove Professor J. S. Higgins Dr R. W. Richards Dr R. A. L. Jones New experimental methods and new theoretical and computational techniques have recently led to great progress in understanding the difficult but technologically important problems associated with the conformation of polymer molecules at surfaces and interfaces. The purpose of this Discussion is to bring together experimentalists and theoreticians working towards a molecular understanding of polymers at surfaces and interactions to survey the progress in the area to date and to indicate future directions of research. The meeting will attempt to bring a unified approach to the problem, encompassing problems of the structure of surfaces and interfaces in polymer melts, the conformation of polymers at solidAiquid and liquid/liquid interfaces, and extensions towards more complicated biological systems. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, Piccadilly ,London W1V OBN.THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 99 Vibrational Optical Activity: from Fundamentals to Biological Applications University of Glasgow, 19-21 December 1994 Organising Committee Professor L. D. Barron (Chairman) Dr A. F. Drake Dr D. L. Andrews Professor R. E. Hester Professor A. D. Buckingham Traditional optical activity measurements such as CD are confined to the visible and near-ultraviolet spectral regions where they provide stereochemical information on chiral molecules via polarized electronic transitions.Thanks to prompting from theory and new developments in instrumentation, optical measurements are now being made in the vibrational spectrum using both infrared and Raman methods. Studies over the past decade on a large range of chiral molecules, from small organics to biological macromolecules, have demonstrated that vibrational optical activity opens up a whole new world of fundamental studies and practical applications undreamt of in the realm of conventional electronic optical activity. The meeting seeks to bring together experimentalists and theoreticians to discuss the current and future experimental possibilities and the development of theories, including ab initio computational methods, which can relate the observations to stereochemical details.The increasing importance now being attached to molecular chirality and solution conformation in the life sciences should also encourage the partipation of biomolecular scientists. The preliminary programme may be obtained from Mrs Angela Fish, The Royal Society of Chemistry, Burlington House, London W 1V OBH. iv THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 100 Atmospheric Chemistry: Measurements, Mechanisms and Models University of East Anglia, Norwich, 19-21 April 1995 Organising Committee : Professor I. W. M. Smith and Dr J. R. Sodeau (Co-chairmen) Dr R.A. Cox Dr J. C. Plane Dr J. Pyle Professor F. Taylor The priority now given by national governments to the study of atmospheric science confirms that our understanding of global climate and compositional changes depends upon measurements in both the laboratory and the field. The data obtained by the experimentalists are then applied by modellers who provide the most significant input into legislative controls on pollution matters. However there have been few opportunities for laboratory and field workers along with the modelling community to attend an "interdisciplinary" discussion in which overall progress in our understanding of specific atmospheric problems is assessed. The object of this discussion is to bring together the researchers in the diverse disciplines that make up atmospheric chemistry so that their individual results and conclusions can be communicated to each other.Some of the key issues to be discussed will include: ozone balances in the atmosphere; heterogeneous processes; the interaction of chemistry and dynamics in determining atmospheric composition and change. Particular reference will be made to the input of data to global models from the use of satellite, airborne and ground-based instrumentation. Contributions are invited for consideration by the Organising Committee covering topics within the area of chemistry, dynamics and modelling in the lower and upper atmosphere. Abstracts of about 300 words should be submitted by 31 May 1994 to: Professor I.W. M.Smith OR Dr R. J. Sodeau School of Chemistry School of Chemical Sciences University of Birmingham University of East Anglia Edgbaston, Birmingham B15 2lT UK Norwich NR4 7TJ, UK Full papers for publication in the Discussion volume will be required by December 1994. THE ROYAL SOCIETY OF CHEMISTRY, FARADAY DIVISION, GENERAL DISCUSSION 101 Gels Paris, France, 6-8 September 1995 Organising Committee: Dr J. W. Goodwin (Chairman) Dr R. Audebert Dr R. Buscall Professor M. Djabourov Dr A. M.Howe Professor J. Livage Professor J. Lyklema Professor S. B. Ross-Murphy During the last few years there has been an increase in both theoretical and experimental work on gels as new techniques have been applied to a wide range of gelling systems.Typical of these are gels formed from polymers by both physical and chemical interactions as well as gels formed by inorganic and surfactant systems. The meeting will deal with the structure and dynamics of gels with the latter heading covering both swelling and rheological behaviour. Mixed systems such as polymer/surfactant and polymer/particle gels will also be discussed. The Discussion will bring together experimentalists and theoreticians interested in different types of gelling systems and encourage them to interact and assess the current scene and provide a benchmark for future developments. Contributions are invited for consideration by the Organising Committee. Titles and abstracts of about 300 words should be submitted by 30 September 1994 to: Dr J.W. Goodwin,School of Chemistry, University of Bristol, Cuntock's Close, Bristol, BS8 1 TS, UK Full papers for publication in the Faraday General Discussion 101 volume will be required by May 1995. V The Royal Society of Chemistry Younger Chemists Committee Pre-Doctoral Chemistry Symposium 1994 Autumn Meeting, University of Glasgow 6th September 1994 CALL FOR PAPERS The Younger Chemists Committee are organising a Pre-Doctoral Symposium as part of the RSC's Autumn Meeting, to be held at The University of Glasgow from 6-9th September 1994, There will be four parallel sessions for oral presentations, plus a poster session, reflecting the themes adopted by the following Divisional symposia at the Autumn Meeting:- Analytical:-Analytical Challenges in Toxicology and PollutionI Dalton:-Diversity in Co-ordination Chemistry Faraday & Macro:-Reactions and Mechanisms for Fine Chemicals in Heterogeneous Catalysis, The Organic and Physical Chemistry of Macromolecules, Perkin:-Organic Chemistry: Synthesis and Mechanisms, Postgraduate and young industrial chemists, aged under 30, are invited to submit abstracts for consideration as oral or poster presentations, Participants whose contributions are accepted will not be expected to pay the registration fee of €20, Papers covering topics not included in the theme of the Au umn Meeting are equally welcome for consideration, Anyone wishing to contribute a paper or poster, should submit a itle and abstract (ca, 100 words) as soon as possible to:-Dr John F Gibson Secret a ry (Scientific) The Royal Society of Chemistry Burlington House London W1V OBN Tel:-071-437 8656 Organised In conjunction with the West of Scotland Section of The Royal Society of Chemistry vi

ISSN:0956-5000    

DOI:10.1039/FT99490BP087

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1197-1204 Investigation into the Kinetics and Mechanism of the Reaction of NO, with CH, and CH,O at 298 K between 0.6 and 8.5 Torr: Is there a Chain Decomposition Mechanism in Operation? Peter Biggs, Carlos E. Canosa-Mas, Jean-Marc Fracheboud, Dudley E. Shallcross and Richard P. Wayne* Physical Chemistry Laboratory, South Parks Road, Oxford, UK OX1 302 The reactions CH, + NO, +products (l), and CH,O + NO, -+ products (2),have been studied using a flow system at T = 298 K and at pressures between 0.6 and 8.5 Torr. The laser-induced fluorescence (LIF) technique was used to detect CH,O and multi-pass optical absorption to detect NO,. The chemical systems were studied as a pair of consecutive reactions; however, a simple analytical treatment was not sufficient to describe them because CH302 was formed as one of the products in the major channel of reaction (2).This species also reacts with NO, regenerating CH,O. Use of a numerical model to correct for this regeneration process allowed rate parameters of k, = (3.5+_ 1.0) x lo-'' cm3 molecule-' s-' and k, = (2.3 & 0.7) x lo-', cm3 molecule-' s-' to be determined at 2.4 Torr. There is no pressure dependence observed for reaction (1) between 1 and 2.4 Torr, but the possibility of a slight pressure dependence for reaction (2) exists. These pressure effects are examined using the semi-empirical quantum RRK method. The importance of the nitrate radical (NO,) as a night-time oxidant in the troposphere has become apparent in recent years.' Although the two reactions CH, + NO, -,products (1) CH,O + NO, +products (2) would not be expected to be significant sinks for methyl and methoxyl radicals in the troposphere, where reactions with molecular oxygen CH, + 0, + M +CH,O, + M (3) CH,O + 0, -+ HCHO + HO, (4) are by far the most important reaction pathways, they are nevertheless worthy of study.First, they are examples of radical-radical reactions where the possibility of more than one product channel exists. Secondly, a knowledge of the product channels and rate parameters for these reactions is essential in the interpretation of our laboratory studies of the interactions of the nitrate radical with CH,O,, which may itself be involved in atmospheric chemistry (see following paper)., No kinetic results have been reported previously for either reaction (1) or reaction (2).Experimental The apparatus is shown in Fig. 1. It is similar to that described in detail elsewhere., A conventional discharge-flow apparatus is used with a double sliding-injector arrangement, coupled to a fluorescence cell. An optical multi-pass absorp-tion cell (12 passes, base path 10 cm) and a quadrupole mass spectrometer were incorporated in the flow tube downstream of the LIF cell. Nitrate radicals were prepared by the reaction of fluorine atoms with dry nitric acid F + HNO, +HF + NO, (5) and detected by optical absorption at Iz = 662 nm.4 An effec-tive absorption cross-section was determined experimentally for NO, [o = (1.1 & 0.1) x lo-'' molecule cm-,] via the titration of NO, with NO NO, + NO -+ 2N0, (6) from which absolute concentrations of NO, could be assign- ed.The minimum detectable [NO,] for a signal-to-noise ratio of unity with a 10 s integration time was ca. 10" mol-ecule cm-,. Experiments were performed at T = 298 K and between 0.6 and 8.5 Torr total pressure, with helium as the carrier gas. The NO, was maintained in excess over the other reactants. Initial concentrations of NO, were typically in the range (0.5-3.5) x lo', molecule cm-, and the organic rad- icals were present initially at concentrations of (0.3-5) x lo', molecule ern-,. For the investigation of reactions (1) and (2), methyl radicals were prepared by the reaction of F atoms with CH, . Methyl radicals reacted rapidly with NO, forming CH,O (see later) in the reaction CH, + NO, +CH,O + NO, (14 The methoxyl radicals so produced could further react with NO, to form products CH,O + NO, -+ products (2) and a consecutive reaction sequence was established in a similar fashion to that observed when NO, reacts with methyl radicals., Thus, by monitoring the concentration- time profile of the methoxyl radical by the LIF technique, the rate coefficients k, and k, could be determined.The details of the LIF detection of the methoxyl radical are described else-where., For calibration purposes, and in some kinetic experiments, we generated CH,O directly in the sliding injector via the reaction of fluorine atoms with methyl nitrite' F + CH,ONO +CH,O + FNO (7) and used this system to study reaction (2).Mass spectro- metric analysis showed that the signal from the species FNO at m/e = 49 remained essentially constant while CH,O decayed, indicating that FNO chemistry does not interfere. Materials Nitric acid (BDH, 99.9%) was dehydrated by sulfuric acid (BDH, 99.5%) in a 1 :2 volume-to-volume mixture and held at ca. 258 K. Helium (BOC) was passed through two traps held at 77 K containing molecular sieve 4A (BDH) to remove water and an OXISORB cartridge (Messer Griesheim) to remove oxygen. Methane (BOC) and fluorine (5% in He) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 He optical absorption system I I 1 I +to flow Pump A1 microwave discharge He excimer pumped dye laser reactant at injector A1 B1 reaction HNO, CH4 CH,ONO F + HNO, +HF + NO, F + CH, +CH, + HF F + CH,ONO --$ CH,O + FNO ~~ Fig.1 Experimental arrangement of the flow tube and detection system. Sources of radicals are indicated in the table. were used without further purification. For the experiments using deuteriated reagents, the chemicals used were : helium (Messer Griesheim), CD, (MSD isotopes, 99% D) and fluorine (5% in He). Methyl nitrite (CH,ONO) was synthesised by the dropwise addition of sulfuric acid (33% in H20, 150 ml) onto a 1 :1 mixture of water and methanol (20 ml each) and sodium nitrite (25 g, Aldrich) at 273 K. The brown methyl nitrite gas produced was pumped through two traps, the first containing CaCO, powder (Aldrich) and the second contain- ing KOH pellets (Aldrich) to remove any residual acid.The gas was then trapped in a cold finger held at 196 K. The resulting yellow liquid was stored in the dark at this tem- perature until required. As will appear later, it is essential to minimise residual methanol in the methyl nitrite. To purify the CH,ONO, it was vacuum distilled from 196 K to 77 K several times (typically three) until the CH,OH impurity, as determined mass spectrometrically, was less than 5%. Results and Their Analysis Reaction of CH,O with NO, using CH,ONO as the Source of CH,O A preliminary investigation of the kinetics of reaction (2) was performed in which CH,O was generated uia the reaction of fluorine atoms with methyl nitrite’ in reaction (7). At first sight, this method seemed to be successful and a good com- bined second-order plot was obtained at 1.6 Torr total pres- sure (see Fig.2), which gives the result k, = 1.6 x lo-’, cm3 molecule-’ s-’. However, we observed anomalous effects when NO, concentrations were lower than 8 x 10l2 mol- ecule ern-,. For these concentrations, rather than a drop in CH,O signal following addition of NO,, an increase occurred. We attributed this effect to the presence of the hydroxymethyl radical (CH20H) and its reaction with NO,. The source of CH,OH was the reaction697 F + CH,OH +CH,OH + HF (8) the methanol being an impurity in the methyl nitrite samples. When methanol alone was used as a source of CH,O, these anomalous effects were more pronounced, confirming our hypothesis. NO, may react with CH,OH to form CH,O directly, or the removal of CH,O in reaction with CH,OH may be suppressed.It is inappropriate to speculate further about this phenomenon, but the value obtained for k, at 1.6 Torr should only be treated as approximate, and an alterna- tive source of CH,O was sought. 0.01 0.02 0.03 0.04 time,/s Fig. 2 Plot of ln([CH,0]o/[CH,0])/~03]o as a function of time. P = 1.6 TOR. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Reactions of CH, and CH,O with NO, using the Reaction between CH, and NO, as the Source of CH,O We discovered an excellent alternative source for the gener- ation of methoxyl radicals, which was a branch of reaction (1) itself CH, + NO, -+ CH,O + NO, (14 Using this source, reactions (1)and (2) were studied as a pair of consecutive reactions.With NO, in excess, both processes (1) and (2)could be treated as first order. It was assumed that, for both methyl and methoxyl radicals, some first-order losses independent of NO, could occur, e.g. on the walls. With these rate coefficients for losses identified as kcH3and k;,,, ,the total first-order loss can be written k; = k& + k,[NO,] k; = ~H,O+ k,"O,I (11) The concentration-time profile for CH,O can therefore be expressed as k; aCCH310CCH301, = ki -k; [exp( -k; t) -exp(-kit)] (111) where kia(=k,,[NO,]) denotes the specific channel forming methoxyl radicals and [CH,], is the initial methyl radical concentration. Eqn. (111) could then be used to fit the experimentally derived LIF signal from CH,O as a function of time.We use this equation as the basis for analytical methods of obtaining the rate constants, but, as we shall show shortly, a numerical correction procedure had to be adopted because of complications in the reaction system. A non-linear least-squares fitting procedure was applied, with k;,[CH,],, k; and k; being the three parameters optimised. Fig. 3 shows a set of typical profiles for [CH,O] and the results of the three-parameter fit. Note that the absolute scale for [CH,O] emerges only as an outcome of the modelling procedures to be presented later, but has no bearing on the three-parameter fit, which can use arbitrary units for the cal- culations. The expected rise and decay indicative of consecu- tive processes is evident.Values of k; and k; obtained from the fitting procedure were plotted against [NO,l0 to yield the second-order rate constants k, and k, . There is no ambiguity about which rate constant is the larger, since we already have 14 12 mI 5 10 2 0.005 0.01 0 0.015 0.020 0.025 time/s Fig. 3 Concentration-time profiles for [CH,O] showing the results of the three-parameter fitting in the CH, + NO, system. P = 2.4 Torr. (+) [NO,], = 4.6 x lo'* molecule an-,; (V)[NO,], = 1.z x lo1, molecule cm-,; (m) mO,], = 1.7 x 10'' molecule cm- . The results obtained from numerical modelling are almost identical (see text). an approximate value for k, obtained from the experiments with CH,ONO. An alternative method employed to analyse the CH30 concentration-time profiles was to use just the decay part of the curve to determine k;.Provided that k; is much larger than k;, and at a sufficiently long time t, eqn. (111) can be rewritten ln[CH,O], = In( k;.CCH3k; 1o)-k; t and k; can be determined. Then a two-parameter (k;,[CH,], and k;) fit of the data using the calculated k; could be per- formed to determine k;.The two methods yielded essentially the same rate constants for k; and k;. However, for some experiments where there were not enough points in the decay part of the curve, the first method was used exclusively. In those experiments where the shortest contact time had already exceeded the time for the maximum [CH,O] to be reached, the second method was used.At a pressure of 2.4 Torr, the three-parameter fit could always be used. This pres- sure is particularly important to us, because it is the same as that used in the experiments described in the next paper2 and in support of which the present work was a necessary precur- sor. We shall show in the subsequent paragraphs that the rate constants must be corrected. In this discussion, several modifications to the pseudo-first-order rate constants, k', and k;,will be needed. We shall adopt a superscript 'exp' to indi- cate uncorrected experimental rate coefficients ; other super- scripts for rate constants corrected for secondary reactions or diffusion, or extracted by numerical modelling, are defined in the footnote to Table 1.The table presents the uncorrected rate Coefficients for kYxP and k;'xPderived at 2.4 Torr (columns 4 and 7). This type of approach has been employed previously3 to good effect for the reactions of NOz with CH, and CH,O. There were, however, two important differences between the NO, and the NO, systems. First, the maximum concentration which could be attained for NO, was an order of magnitude less than for NO, ([NO,],,, = 3.5 x lOI3 mol-ecule ern-,); thus, in the NO, case, methyl radical loss via its self reaction could not be neglected. Secondly, at long contact times in the NO, system, the LIF signal for CH,O decayed to zero, whereas in the NO, experiments there was a distinct CH,O signal still present even at contact times as long as 120 ms.In fact, this signal was virtually constant after 80 ms. This second phenomenon is demonstrated in Fig. 4, which shows a typical experimental concentration-time profile for CH ,O together with a fit from a numerical model (see later). It is evident that some process is in operation that regenerates CH,O. A probable explanation for this regeneration is that the products of reaction (2) are predominantly CH,O, and NO2 CH,O + NO, -+ CH302+ NO, (24 and that CH,O, itself reacts with NO, reforming CH,O CH,O, + NO, -+ CH,O + NO, + 0, (9) The regeneration process appears to be efficient, as demon- strated by the very small decay of CH,O seen at long contact times, which itself points strongly to reactions (2a) and (9) being the major channels in the reaction of CH,O and CH302 with NO,.Model calculations show that the fraction of CH,O radicals passing through the reactions that regener- ate these radicals is between 0.6 and 0.7 for t > 60 ms. To test the hypothesis that CH,O was regenerated, a set of experi- ments was performed to look specifically for CH302 as a product, as we shall discuss later. Regeneration of CH,O makes simple application of the three-parameter fitting J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Values of k; and k2 at P = 2.4 Torr P/Torr [N0,],/10'3 molecule cm-, A[NO,]/~O,], k':xP/s-' k?/s-l k';um/s-l k;'"P/s-' k;d"'/s -' 2.37 0.81 0.18 595 368 324 69 72 2.40 0.46 0.20 364 212 189 60 62 2.40 0.58 0.14 370 252 220 61 63 0.14 442 303 307 74 772.40 0.83 2.40 0.80 0.14 366 250 240 78 81 2.40 1.24 0.12 593 440 397 74 77 2.40 1.64 0.09 856 675 656 69 71 2.40 2.27 0.02 863 88 92 2.31 1.20 0.14 496 341 336 74 77 2.31 1.92 0.12 1047 766 730 103 108 2.31 1.42 0.10 622 482 383 82 86 2.38 0.94 0.11 520 394 329 68 71 2.38 1.23 0.12 486 356 332 82 86 2.38 1.53 0.10 717 547 459 71 73 2.47 2.28 0.06 95 100 2.39 1.21 0.12 570 412 424 71 74 2.39 0.63 0.16 293 192 189 54 56 2.43 1.67 0.13 719 508 451 83 87 2.40 1.97 0.14 89 93 2.40 1.28 0.16 592 39 1 384 64 66 2.38 1.35 0.13 567 400 378 69 72 2.38 0.68 0.15 364 246 204 54 55 Column 3 displays the values of A[NO,]/[NO,], averaged over all contact times.Column 4 shows the values of k; derived from applying the three-parameter fitting method to the experimental data.In column 5 are the corrected values of k', (see text). In column 6 are the values of k', derived from application of the numerical model to the experimental data. Column 7 shows the values of k; derived from application of the numerical model to the experimental data. Column 7 shows the values of k; derived from either a ln(CH,O) us. time plot (see text) or the three-parameter fitting method. Column 8 shows the values of k2 corrected for axial diffusion and radial concentration gradients. method as used previously for the NO2 system subject to error., We therefore adopted a correction procedure in which a numerical model was used to refine the results of the three- parameter fit in an iterative way.We prefer this procedure to straight fitting to a numerical model. Preliminary trials indi- cated that there was no advantage in using such a model to fit the data from our experiments at relatively short contact times (t < 25 ms). It is not possible either to distinguish between the reaction of CH,O with NO, to form CH302 and other losses of CH,O, or to define a value for the rate coefficient for reaction between CH302 and NO,. The numerical integrations for the model were performed using a program written in BASIC that employed a second-order single-step backward-differentiation method. 5 4E4 n \ _/------_ I 0.02 0.04 0.06 0.08 0.10 time/s Fig. 4 Experimental (CH,O) and modelled (CH,O, CH,O,) concentration-time profiles for long contact times in the CH,+ NO, system.P = 0.6 Torr; pTO,l0 = 2.5 x lo', molecule ~rn-~. (D)Expenmental [CH,O]; (-) modelled [CH,O]; (---) model-led [CH,02]. Correction Procedures Data for the numerical model are given in Table 2. Fitting was achieved largely by eye, assisted by examination of mean-square deviations. The approximate values of k,, k2 and k, so derived are then used as the starting point for a correction procedure based on the numerical model. This procedure consisted of first generating simulated concentration-time profiles of [CH,O] for a variety of start- ing conditions, including input values of k; and k;, called here k';'" and k';'". We then ran the three-parameter fit on these generated data to recover the values of ,;,Utand kpt.In all experimental runs, we observed a drop in [NO,] on addi- tion of methyl radicals to the system (Table 1).This drop, identified as ACNO,], was usually about 10-15% of the [NO,] in the absence of CH, ([NO,l0). A simple empirical relationship was found relating R = k'li"/k'out to AINO,]/[NO,]o. This relation is R = (0.97 f0.02) -(1.86 & 0.12) x A[NO,]/[NO,]o (V) R was used to modify the values of k;exp, obtained from the three-parameter fit to the experimental data. The modified values for k; (termed k'y)at 2.4 Torr are shown in Table 1, column 5. Fig. 5 shows k'? (= R x l~':"~)plotted as a func- tion of [NOJ0. Although there are generally few points on the rise portions of the concentration-time profiles, the time dependence of the maximum of the [CH,O] provides a good definition of k;,as indicated by the relatively small scatter in Fig.5. Indeed, the statistical error on the slope of this figure is k0.24 x lo-" cm3 molecule-' s-l (95% confidence limits), but an additional error arises from uncertainty in the zero of time that is itself due to mixing effects and axial diffu- sion. We therefore quote k, as (3.5 f1.0) x lo-" cm3 molecule-' s-'. The intercept in Fig. 5 is zero, the reason for which is that the correction procedure corrects for any first- order losses that are independent of [NO,]. Correction of the rate constant for reaction (2) proceeds in a similar manner, but here there appears to be no simple factor to be applied to the pseudo-first-order rate coefficient J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Rate parameters used in the numerical model reaction k/cm 'molecule -' s -' number ref. CH, + NO, CH,O + NO, 3.3 x lo-" (14 a4 CH, + CH, -+ CH,CH, 4 x lo-" (10) 8 CH,O + CH,O -,products 1.0 x lo-" (1 1) 9 CH, + CH,O +products 4 x lo-" (12) b CH,O -,products 25-50 s-' (13) see text CH,O + NO, -,CH,O, + NO, CH302+ NO, +CH,O + NO, + 0, 2-3 x lo-', (24 see text 1 x lo-', (9) see text CH, + NO, -+ CH,O + NO 2.3 x lo-" (14) 3, 10 CH,O + NO, products 2.0 x lo-', (15) 3, 11, 12 CH,O + NO -,products 5.0 x lo-', (16) 13 NO,+NO,+M-+N,O,+M 4.0 10-14 (1 7) 14 NO, + NO -,2N0, 3.0 x lo-" (6) 15 a The numerical model yields a value k, = (3.3 f0.9) x lo-" cm3 molecule-' s-'; the corresponding first-order rate constants are listed in Table 1.For reaction (12) the rate constant was estimated to be twice the geometric mean of k,, and kl,. k;. Instead, we plot the values of Py' (generated by applica- the evaluation of the rate constant k; by the analytical tion of the three-parameter fit to the simulated data) against method. The only way of extracting kinetic parameters from [NO,] to obtain a predicted value of the second-order rate these data was to use the numerical model. It was possible to constant, ko;l*,and compare k? with this value to provide the distinguish clearly between the numerical fits to the experi- correction. The relation is k? = 1.1 x ko;lf. Values of k;'"p mental data at these long contact times (up to 120 ms), for were obtained from the three-parameter method or from values of k, in the range (1.5-2.5) x lo-'' cm3 molecule-' direct logarithmic analysis at relatively long contact times.s-'. Similar calculations were carried out for the experiments These values were corrected for radial and axial diffusion at 1.01 and 1.40 Torr. For the higher pressures, 2.4-8.5 Torr, using the standard methods of Walker,16 Brown1' and the numerical model produced very good fits to the experi- Keyser" to yield the rate constant k;diff. Fig. 6 is a plot of mental data using the values of k, shown in Table 3 (column k;diff against [NO,], for three pressures, and is the source of 4).In making this fit, k, is used as an adjustable parameter the uncorrected value of k, . Table 3 collects the uncorrected and a value for this rate constant can therefore also be and final corrected values for k, at each pressure. At the derived. The value obtained for k, is (l.O?A:!) x lo-'' m3 lowest pressure (0.6 Torr), the available linear flow velocity molecule-s-'. The error limits indicate the highest and was not sufficient to give a range of contact times suitable for lowest possible rate coefficients that could be used to fit the 200800 U 150600 c cI I9 5100400 0-?m* 50 V I 0.5 1.o 1.5 2.0 1 2 3 4 molecule ~m-~[N0,],/1 013molecule ~m-~ [N03]0/1013 Fig. 5 Corrected pseudo-first-order rate coefficient (see text) for the Fig.6 Pseudo-first-order rate coefficient (derived from the three- reaction CH, + NO,, plotted as a function of [NO,], . (V)1.0; (A) parameter method) for the reaction CH,O + NO,, plotted as a func- 2.4 Torr. tion of WO,], .(V)1.0; (0)1.4and (0) 2.4 and (a)5.4 Torr. Table 3 Summary of the values obtained for k, at various pressures ~ ~___ pressure/Torr no. of experiments k,/10-I2 cm3 molecule-'s-' k,/10-I2 cm3rnolecule-'s-' k,/10-cm3 molecule-'^-^ 0.60 6 2.2 f0.5 1.01 9 1.6 f0.3 1.7 f0.3 2.1 0.6 1.40 6 1.8 0.6 2.0 f0.6 2.1 0.6 2.40 22 2.1 f0.7 2.3 f0.7 5.44 10 3.0 f1.2 3.3 f1.2 8.50 6 3.3 f1.5 3.6 f1.5 Column 3 shows the values of k, derived from the three-parameter fit corrected for axial diffusion and radial concentration gradients. Column 4 shows the values of k, from column 3 corrected by the numerical method.Column 5 shows the values derived for k, by fitting to the numerical model. 0.01 0.02 0.03 0.0 4 time/s Fig. 7 Mass-spectrometric determination of the relative concentra- tion of CD,O, in the reaction of CD, with NO,. (m) Experimental points; (-----) a fit using the numerical model (see text). experimental data. However, the fits using these limiting values are not as good as those employing the centre value, requiring values of k,/k, which are incompatible with the experiments presented in the next paper., Identification of CH,O, We cannot detect CH,O, directly in our system. In experi- ments where CH,O, is produced directly (see subsequent paper),2 titration with NO CH,O, + NO +CH,O + NO, (18) can be used as an indirect method of monitoring the peroxy radical.However, in the present studies the NO would react mainly with NO, and CH,O so that the method is not satis- factory. An alternative series of experiments was used to demonstrate the occurrence of reaction (2a).A discharge-flow system at the University of Kiel (Germany) equipped with a quadrupole mass spectrometer that had a high sensitivity towards the peroxy radical CH,O, (1 x lo1' molecule cm-,) was used for this purpose. In these experiments, CD, was substituted for CH,. Unambiguous assignment of CD,O, at its parent ion (m/e= 50) was possible, whereas the CH,O, peak (m/e= 47) suffered from strong interferences from sec- ondary peaks of other species in the system.The deuteriated peroxy radical was indeed identified; it arises from the reac- tion sequence F + CD, +CD, + DF (19) CD, + NO, +CD,O + NO, (20) CD,O + NO, +CD302+ NO, (21) A typical experimental run showing the build up of CD,O,, together with the calculated [CD,O,] from the numerical model, is shown in Fig. 7. We are confident, therefore, that the inclusion of reaction (2a) in the numerical model is justi- fied. Discussion Reaction between CH, and NO, It is clear that the reaction between CH, and NO, is fast; the major channel appears to be process (la). It is also evident that the reaction does not show a significant pressure depen- dence, at least over the range 1.0-2.4 Torr (see Fig.5). In a previous study,, we investigated the pressure dependence of the reaction of NO, with CH, and CH,O and used the semi- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 empirical quantum RRK (QRRK) methodlg to rationalise our observations. If the same approach is adopted here, we must first look at the possible products which arise from the formation of the [CH,-ONO,]* energised complex. There are four possible channels, illustrated by the energy diagram for this reaction system in Fig. 8; simple bond fission of the complex can either regenerate the reactants or yield CH,O + NO, depending on whether the C-ON or CO-N bond breaks. Similarly, the energised complex can undergo exten- sive bond rearrangement and fission to form HCHO and HONO; and finally, the complex can be stabilised to form an adduct, which in this system is likely to be CH,ONO,.The QRRK method allows the calculation of the microcanonical rate constants for each particular channel for a given total internal energy of the energised complex. The data required for this calculation are the high-pressure Arrhenius pre- exponential factor (A;) and the activation energy (E') for each process i; the parameters employed are summarised in Table 4. No information exists for the dissociation of CH,ONO, to CH, and NO,. The activation energy was therefore taken to be the difference between the heat of for- mation of CH,0N02 and the combined heat of formation of CH, and NO, ;the A factor was estimated from A factors for similar simple bond fission reactiom2' Table 5 shows the partitioning of the four channels at four pressures.It is clear that channel (la) should dominate at pressures up to 10 atm and that the stabilised adduct (CH,ONO,) would only become a significant channel beyond this pressure. This result is consistent with our experimental conclusion that channel (la) is the only branch of reaction (1) in our system and indi- cates that a pressure dependence is not expected. Reaction between CH,O and NO, As in the case of reaction (l),several possible channels CH,O + NO, +CH302+ NO, (24 CH,O + NO, + HCHO + HNO, (24 CH,O + NO, + M +CH302N02+ M (2c) exist for reaction (2). As pointed out earlier, the extent to which CH,O is regenerated in the pair of reactions (2) and (9) I CH3+N03-200.-I100 -r I-z2 0-5 -100 -CH,ONO, \L-200t-HCHO+HONO Fig.8 Energy diagram for the reaction between CH, and NO,. The energies on which this diagram is based, and the justifications for their adoption, are presented in Table 4. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Parameters used in the QRRK calculation for reaction (1) reaction A1s-l E/kJ mol-' (v)$cm-m ref. aCH,ONO, + CH, + NO, 1.0 x 1oI6 337.0 972.4 29CH,ONO, -+ CH,O + NO, 3.2 x 1015 166.5 972.4 15 22 CH,ONO, + HCHO + HONO 3.3 1013 151.0 972.4 13 23 The A factor was estimated" and E was ~alculated'~ from ArH(CH,) + ArH(NO,) -A,H(CH,ONO,). (v), is the geometric mean frequency for CH,ONO,, estimated from the vibrational data of Bock et al." m is the critical number of quanta required for reaction, i.e.E/hv + 1 2 rn 2 E/hv; here the lower bound is stated. The number of oscillators, for CH,ONO, is 18. Table 5 Fractional contributions of different channels obtained in the QRRK calculations on the CH, + NO, system Platm process 0 1 10 100 CH, + NO, CH,O + NO, HCHO + HONO ca. 0 0.96 0.04 ca. 0 0.95 0.04 ca. 0 0.90 0.04 ca. 0 0.58 0.03 CH,ONO, (stabilised) ca. 0 0.01 0.06 0.39 indicates an overall efficiency of 0.6 to 0.7, and thus the lower limit for the fractional contribution of reaction (2a)is also in this range. An interesting feature of reaction (2) is the possible weak pressure dependence observed up to 8.5 Torr (Table 3).By analogy with the corresponding reactions of NO, CH,O + NO, --+ HCHO + HONO (19a) CH30 + NO, + M -,CH,ONO, + M (19b) where the pressure-dependent channel (19b) dominates3,' '9' above a pressure of 1 Torr, we might expect reaction (2) to show the same characteristics. In addition, the peroxynitrate (CH,O,NO,) formed in reaction (24 has three more vibra- tional modes than the corresponding alkyl nitrate (CH,ONO,) formed in reaction (19b), so that, for a given fractional excess internal energy and pressure of the bath gas, the stabilisation of a peroxynitrate energised complex relative to redissociation would be expected to be greater than that for the alkylnitrate.Once again, the QRRK method was used to investigate this hypothesis. The data relevant to the calcu- lation are summarised in Table 6. There were no data in the literature for the high-pressure dissociation of methyl per- oxynitrate to form either CH,O + NO, or HCHO + HONO, ; the A factor was therefore estimated from the kinetics of similar reactions.20 The activation energy for the first of these channels was estimated from heats of forma- tion," and for the second an estimate was made based on the dissociation of CH,ONO, to HCHO + HONO.,, The QRRK calculations show that complex bond fission of the energised complex leading to HCHO + HONO, is negligi- ble, even when the largest possible A factor is used (Table 7). The calculations show that the relative importance of stabili- sation of the energised complex is also small.The redissociation channel reforming reactants is negligible even when the assumed A factor is some ten times greater than that for channel (24. A measurable pressure dependence of rate of reaction is only expected if changes in bath-gas con- centration alter the relative rates of reaction into the various channels as a consequence of the redistribution of the inter- nal energy. As Table 7 shows, no channel other than (24 appears to contribute in the pressure range of our experi- ments. No dependence of k, on pressure is thus expected. We note that if we use the smallest possible activation energy for redissociation (i.e.the minimum possible energy difference as dictated by the error limits quoted for the heats of formation), we find that at zero pressure a fraction of 0.1 of the energised complexes formed do redissociate to reactants and 0.9 form CH,O, + NO,.There is then the possibility of a small pressure effect. Although there is some tendency for k, to increase with pressure (Table 3), we believe that the experimental uncertainties in the individual rate constants do not allow us to be more dogmatic at present. The real problem is that the QRRK method cannot be applied in cases where the energy difference between different possible product channels is small enough to make the predicted dif- ferences in rate coefficients comparable with the inherent errors in the calculation of those coefficients introduced by the approximations.This problem is compounded where there are uncertainties in the thermochemical data. We would like to express our gratitude to the NERC (grant GR3/7359) for support for this project, and to the CEC and the Institute Franqais du Petrole, under whose auspices various parts of this and related work were carried out. We are indebted to Professor R. N. Schindler for the use of the discharge-flow mass spectrometer apparatus in Kiel and to Tim Jungkamp for his assistance with the experiments on CD, carried out with this equipment. We thank Richenda Table 7 Fractional contributions of different channels obtained in the QRRK calculations on the CH,O + NO, system Pporr process 0 1 10 760 CH,O,NO, -+ CH30,N0, -+ CH,O + NO, CH30, + NO, ca.0 0.98 ca. 0 0.98 ca. 0 0.95 ca. 0 0.80 CH,O,NO, -+ HCHO + HONO, 0.02 0.02 0.02 0.01 CH,O,NO, (stabilised) ca. 0 ca. 0 0.03 0.19 Table 6 reaction A/s- E/kJ mol- CH30,N0, + CH,O + NO, CH,O,NO, + CH,O, + NO, CH,O,NO, -+ HCHO + HONO, 1 x 1017 1.1 x 10l6 1 x 1015 130.0 87.8 100.0 a The A factor was estimated,' and E was ~alculated'~ from Af H(CH,O) + Af H(N0,) and the energy barrier was estimated by comparison with the reaction" CH,ONO, quency for CH,O,NO, ,calculated from the vibrational data of Zabel et aLZ4 Parameters used in the QRRK calculation for reaction (2) (v>,/cm - m ref. 724 15 a 724 12 b 724 11 24 -Af H(CH,O,NO,). The A factor was estimated2' + HCHO + HONO.(v), is the geometric mean fre- 1204 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Connell for preparing the methyl nitrite samples used in this work. D.E.S. would like to thank the SERC for a research studentship. 11 12 J. A. McCaulley, S. M. Anderson, J. B. Jeffries and F. Kaufman, Chem. Phys. Lett., 1985,63, 180. M. J. Frost and I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1990,86, 1751. 13 M. J. Frost and I. W. M. Smith, J. Chem. SOC., Faraday Trans., References 1 R. P. Wayne, I. Barnes, P. Biggs, J. P. Burrows, C. E. Canosa-Mas, J. Hjorth, G. Le Bras, G. K. Moortgat, D. Perner, G. Poulet, G. Restelli and H. Sidebottom, Atmos. Enuiron., A, 1991, 14 15 1990,86, 1757. C. A. Smith, A. R. Ravishankara and P.H. Wine, J. Phys. Chem., 1985,89,1423. W. B. DeMore, S. P. Sander, D. M. Golden, M. J. Molina, R. F. Hampson, M. J. Kurylo, C. J. Howard and A. R. Ravishankara, 25, 1. P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross 2 Chemical Kinetics and Photochemical Data for use in Strato- spheric Modeling. Evaluation number 10, JPL Publication 92-20, and R. P. Wayne, J. Chem. SOC., Faraday Trans., 1994,90, 1205. 1992. P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, A. D. Parr, D. E. Shallcross, R. P. Wayne and F. Caralp, J. Chem. Soc., Faraday Trans., 1993,89,4163. 4 C. E. Canosa-Mas, M. Fowles, P. J. Houghton and R. P. Wayne, 3 16 17 18 19 R. E. Walker, The Physics ofFluids, 1961,4, 121 1. R. L. Brown, J. Res. Natl. Bur. Stand. (US), 1978,83, 1. L. F. Keyser, J. Phys. Chem., 1984,88,4750. A. M. Dean, J. Phys. Chem., 1985,89,4600. J. Chem. SOC., Faraday Trans. 2, 1987,83,1465. 20 S. W. Benson, Thermochemical Kinetics, Wiley, Chichester, 2nd 5 P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross edn., 1976. and R. P. Wayne, in preparation. 21 Ch. W. Bock, S. V. Krasnoshchiokov, L. V. Khristenko, Yu. N. 6 J. L. Durant Jr., J. Phys. Chem., 1991,95, 10701. 7 D. J. Bogan, M. Kaufman, C. W. Hand, W. A. Sanders and B. E. Brauer, J. Phys. Chem., 1990,94,8128. 8 M. J. Pilling, Znt. J. Chem. Kinet., 1989,21,267. 22 23 Panchenko and Yu. A. Pentin, Chem. Phys., 1985,106,69. J. A. McCaulley, PhD Thesis, University of Pittsburgh, 1987. P. Gray, J. F. Grifiths and K. Hasegawa, Int. J. Chem. Kinet., 1981, 13, 817. 9 R. Zellner, D. Hartmann, J. Karthauser, D. Rhasa and G. Weibring, J. Chem. Soc., Faraday Trans. 2, 1988,84,549. 24 F. Zabel, A. Reimer, K. H. Becker and E. H. Fink, J. Phys. Chem., 1989,93,550. 10 F. Yamada, I. R. Slagle and D. Gutman, Chem. Phys. Lett., 1981, 83,409. Paper 3/06976A; Received 23rd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001197

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1205-1210 1205 Investigation into the Kinetics and Mechanism of the Reaction of NO, with CH,O, at 298 K and 2.5 Torr: A Potential Source of OH in the Night-time Troposphere? Peter Biggs, Carlos E. Canosa-Mas, Jean-Marc Fracheboud, Dudley E. Shallcross and Richard P. Wayne* Physical Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QZ The kinetics of the reaction CH,O, + NO, +CH,O + NO, + 0, (1) have been studied at 298 K and at pressures between 2 and 3 Torr of helium using the discharge-flow technique combined with laser-induced fluorescence detection of the methoxyl radical and measurements of the NO, radical using visible absorption. Numerical modelling of the concentration-time profile of CH,O with or without NO, and NO as a titrant has allowed us to show that CH,O is a product of reaction (1) and to derive a rate constant k, = (1.0 f0.6) x cm3 molecule-' s-', at 95% confidence limits.A comparison of the reactivities of NO, and NO, towards the species R, RO and RO,, where R = H or CH,, is given. The implication of reaction (1) in the possible production of OH in the atmosphere at night is discussed. Peroxy radicals (e.g.CH302)are generated during the day in at T= 298 K and at 2.5 Torr pressure, with helium as the the troposphere by photo-oxidation of organic compounds.' carrier gas. Pseudo-first-order conditions were maintained The radicals may persist into the night and it has been sug- throughout, with NO, as the excess reactant. Methylperoxy gested by Platt et al.' that NO, can react with peroxy rad- radicals were generated by allowing oxygen and methane to icals flow through the inner sliding injector and fluorine atoms to CH,O, + NO, +CH,O + NO, + 0, (1) flow through the outer sliding injector.The methyl radicals, thus taking over the day-time r81e of NO which converts produced by the reaction of fluorine atoms with methane, peroxy radicals to alkoxy radicals (e.g. CH,O) by the reac- added to 0, in the reaction tion CH, + 0, + M -,CH302+ M (7) CH,O, + NO +CH,O + NO, (2) forming methyl peroxy radicals in the outer sliding injector. Where NO and NO, levels are low, peroxy radicals will react However, in this low-pressure system it was never possible to with HO, convert all the methyl radicals to CH302via reaction (7).As a result, some CH,O was produced from the reaction ofCH,O, + H02 +CH302H+ 02 (3) residual methyl radicals with CH,O, and will also undergo self reaction CH302+ CH, +2CH,O (8)CH,O, + CH302+products (4) Further, one channel of the self reaction of CH,02 albeit slowly. The alkoxy species (CH,O) formed in reaction (1) can lead to the production of HO, by its reaction with CH,O, + CH3024 2CH,O + 0, (9) 02 9 also contributed to the CH,O signal. The CH302 emerging CH,O + 0, +HO, + HCHO (5) from the tip of the injector therefore always contained some CH,O. In order to monitor the [CH,O,], NO was added to Hall et aL3 and Mellouki et aL4 have shown that the major the system just before the LIF cell (and necessarily in a fixed channel of the reaction between HO, and NO, position with respect to the detector), effecting the conversion HO, + NO, +OH + NO, + 02 (6) of CH302to CH,O is sufficiently fast to effect the conversion of H02 to OH.The CH302+ NO CH,O + NO2 (2)hydroxyl radical is the dominant day-time oxidant but has generally been assumed hitherto to have no sources at night; which was reflected in an increase in the LIF signal from the NO3-mediated oxidation process makes it possible for CH,O. The ratio of [CH,O,] :[CH,O] in the flow could be OH to be formed. determined in this way. Typical (initial) concentrations used In this paper, we describe a study employing a low-pressure in this system were: [F] = 5 x lo', molecule crn-,, discharge-flow apparatus to investigate the kinetics and [CH,] = (2-5) x lo', molecule ern-,, [O,] = (2-5) x 10l6 mechanism of reaction (1).The reaction is of considerable molecule ern-,, [NO,], = 0.4-3.5 x lo', molecule ern-,; potential importance since CH,O, is the most abundant initial concentrations of organic radicals were (0.5-5) x lo', peroxy species in the atmosphere.' molecule ern-,. Oxygen (BOC, 99%) was passed through a This study represents the first direct discharge-flow experi- trap containing molecular sieve 4A (BDH) to remove water. ment in which radicals were generated separately and in Nitric oxide (Messer Griesheim, 99?40)was purified by repeat- which the CH,O product of reaction (1)was identified explic- ed freeze-pumpthaw cycles at 77 K. All other materials and itly.purification procedures used were identical with those described in the preceding paper.6 Experimental Results and Discussion The apparatus used was identical to that described in the Kinetic Data and Their Analysispreceding paper.6 A flow method was employed; CH,O was detected by laser-induced fluorescence (LIF) and NO, by In the study of reaction (l), four signals (identified as A, B, C multi-path optical absorption. Experiments were performed and D) were measured for the methoxyl radical at each J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 contact time. As previously explained (see Experimental section), we always observed a methoxyl signal when gener- ating CH302. Signal A is the total fluorescence from meth- oxyl produced from reaction (8) and (9) in the absence of any added NO or NO,.On addition of NO,, the methoxyl fluo- rescence always increased and the LIF response was termed signal B. This increase was due to formation of CH,O in -mCreaction (1). Addition of NO to the reaction system altered the LIF signals because of the conversion of CH302 to .-CI) v) CH,O in reaction (2). Signal A became signal C, and signal B became signal D. A typical set of signals A, B, C and D is shown in Fig. 1, and the caption specifies the chemical com- ponents used to generate each signal. In principle, of course, addition of NO increases the fluorescence, because CH302 is -converted to CH,O in reaction (2). However, since NO reacts chart motion with NO, and CH,O reacts with both NO and NO, Fig. 1 Chart trace showing an example of the signals recorded in the experiments on the CH,O, + NO, system.The full lines are the NO, + NO +2N0, (10) LIF signals from CH,O: A, for the system CH,O/CH,O, ; B, for the system CH,O/CH,O, + NO, ; C, for the system CH,O/CH,O, CH,O + NO +products (11) + NO; D, for the system CH,O/CH,O, + NO, + NO. The dashed CH,O + NO, -,products (12) line shows the concentrations of NO, for [NO,], = 3.3 x lo', mol-ecule cm-, and ACNO,] = 0.6 x lo1, molecule ern-,. the response of LIF intensity to added NO is complex. All four signals were used in the numerical modelling of these the reaction. The occurrence of reaction (1 3) makes analytical chemical systems (see later) to determine [CH,O,]/[CH,O] treatment of the experimental data impossible, even though for a range of contact times and NO, concentrations. One [NO,] 9 [CH,O,].Other competing and secondary reac- crucial feature should be noted about the typical signals tions, such as the second-order loss of CH,O,, further com- shown in Fig. 1: the [CH,O] increases from signal A to plicate the analysis. For these reasons, the rate constant k,signal B, i.e. on addition of NO, to CH302. This result was derived by numerical modelling using the programdemonstrates explicitly for the first time that NO, reacts with FACSIMILE7 to integrate the differential equations describ- CH302 to produce CH,O in reaction (1) ing the kinetics of the set of reactions displayed in Table 1.CH302+ NO, -,CH,O + NO, + 0, (1) Initial examination of the results of such modelling showed clearly that varying k, and k13 had a complementary effect It is evident that CH302 is regenerated in the step on the predicted [CH,O] concentration. This being the case, it would thus be impossible to extract the two rate constants CH,O + NO, +CH,02 + NO, (13) independently from our experimental data for the CH,O con- As discussed in the preceding paper,6 the efficiency of regen- centrations. However, altering kl, in the model affected k, in eration of CH,O in steps (13) and (1) is 0.6to 0.7. This result such a way that the ratio r = k,/k13 remained constant. The argues against a major channel for reaction (1) leading to approach we adopted for determining k, was therefore to use products such as CH,O, especially as wall losses also remove the value determined in other independent experiments6 for radicals.The effects of adding NO when NO, is present (the k13 in combination with the ratio r obtained here. D signals) are also compatible with virtually all reactions of We always observed some consumption of NO, (ANO,,CH,O, with NO, yielding CH,O. However, we recognise typically 10-20%) on addition of CH,O,, which was obvi- that competing channels might make a minor contribution to ously due to reaction (1) and, to a lesser extent, reaction (13). Table 1 Rate parameters used in the numerical model (see text) reaction k/cm3 molecule-' s-' reaction ref. ~~ CH,O, + NO, -+ CH,O + NO, + 0, CH,O, + NO -+ CH,O + NO, CH,O, + CH,O, +products CH, + 0,+ M-+CH,O, + M CH, + CH,O, -+ CH,O + CH,O CH,O, + CH,O, +CH,O + CH,O + 0, NO, + NO +2N0, CH,O + NO -+ products CH,O + NO, -+ products CH,O + NO, -+ CH,O, + NO, F + CH,+CH, + HF CH, + NO, -+ CH,O + NO, CH, + CH, -+ C,H, (0.6-1.6) x lo-', 7.6 x lo-', (0.9-2.0) x lo-', (3.0-6.0) x lo-" 3.0 x lo-" 4.0 x lo-', 2.3 x lo-" (1.6-3.6) x lo-', 8.0 x lo-" 4.0 x lo-'' 4.0 x lo-" 3.0 x 10-13 1.1 x 10-13 1 2 3 7 8 9 10 11 12 13 14 15 16 this work 9 11 10" 13 11 9 16 8 6 9 6 12 CH, -+ products CH, + CH,O -+ products CH,O + CH,O, +products CH,O + CH,O +products CH,O -+ products CH,O, +products 1 s-l 4.0 x lo-" 3.0 x 1.0 x lo-" 15-50 s-' 3-5 s-l 17 18 19 20 21 22 this work 14 15 this work this work b ~~ ~~_____~~____ a The bath gas in the sliding injector, where the CH,O, was generated, was an approximate 1 : 1 He-0, mixture; the exact rate constant used to model a particular experiment depended on this bath-gas ratio.The geometric mean value was used to estimate this rate constant. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The values of k, and k13 are therefore related to the mea- sured A[N03], so that measurements of this quantity might be used, in conjunction with the values of r already deter- mined, to fix independently the values of the two rate con-stants within the set of experiments described here. However, we regarded the precision of the measurement for ACNO,] to be insufficient for this purpose. Instead, the determinations of ACNO,] were used as a further check that the model was correct.Details of the Numerical Modelling A standardised protocol was adopted for the numerical evaluation of the signals. The procedure for fitting A and B signals is conventional and straightforward, and can be run to give a good match, as judged by the squared deviations, to points at all the contact times simultaneously. For the C and D signals, the integrations must be run for each point indi- vidually, because of the chemical processes initiated by the injection of NO. Our analysis starts with the A signal, which is determined by the chemistry of the organic radicals alone. The LIF calibration factors (checked against the initial F-atom concentration) and the rate coefficients for homo- geneous and heterogeneous reactions of CH,O and CH,O, are adjusted, always within limits imposed by published data.An important check at this point is that the simulated C signal also fits the experimental data well, because a good match indicates that [CH ,O2]/[CH30] has been correctly determined. From this stage, the parameters adjusted so far are regarded as fixed. Next, NO,-initiated chemistry is intro- duced. The only adjustable parameters are k, and k,, in fitting to the experimental points of curve B. Finally, the D signals are calculated as a check, although poorer matches with the experimental data have been regarded as acceptable because of the considerable complexity, and critical behav- iour, of the chemistry when CH,O, CH302, NO, NO, and NO, are all present together.Fig. 2 shows a sample set of experimental data (squares and triangles) and the associated modelled CH,O concentration (solid curves). Fig. 2(a)shows signals A and C, while Fig. 2(b) shows signals B and D. For clarity of presentation, points for signals C and D are given for only three positions of the sliding injector; the modelled curves for these signals show the anticipated changes in [CH,O] from the point of injection of NO. It can be seen, for the values of k, and k,, chosen, that all four signals behave as expected, both as a function of contact time and with respect to the addition of NO3 and NO. The model required as inputs the initial [O,] in the sliding injector and the initial [NO,] in the main flow.Allowance was made for the dilution of the radicals that emerged from the sliding injector. In our implementation of the kinetic modelling, we also started with [Fl0 and [CH,], and regard- ed the various rate coefficients for the initial reactions leading to CH30 and CH,O, as fitting parameters. This procedure really serves to define [CH,O] and [CH30,] as needed to match the A and C signals. We regard this method as rather more satisfactory than the simpler method of varying [CH,O], and [CH3O2IO, because we are forced to adopt realistic values at the outset for the concentration of F atoms and for the rate coefficients. Table 1 shows which rate con- stants, other than k, and k13, were varied, and the limits between which they were altered in the fitting procedures.It should be emphasised again that, once these rate coefficients had been used to fit the A and C signals, they remained unal- tered while fitting the B and D signals. Table 2 shows the relevant experimental conditions and the ratio I (= k1/k13) obtained for each run. In total, model- ling was performed on 18 experiments, from which r was 1207 I I II time/s I I I N ‘j 05 c--. a I 0, I 1 I I 0.02 0.04 0.06 c time/s Fig. 2 An example of experimental data and the associated model- led CH,O concentrations (a)(0)experimental signal A; (A) experi-mental signal C; (b) (m) experimental signal B; (A)experimental signal D. The solid lines are the corresponding modelled values.determined to be (0.43 & 0.09); errors are quoted as one stan- dard deviation. With this ratio r and the value determined6 for k13 = (2.3 & 0.7) x lo-’, cm3 molecule-’ s-l, a value for k, = (1.0 0.6)x 10-l2 cm3 molecule- s-was calculated; the quoted error, at the 95% confidence limits, is the com- bined error in the ratio r and the rate constant kI3. Using these values for k, and k13, it was possible to reproduce the observed consumption in [NO,] (ACNO,]), within the preci- sion of the measurements, for all experiments. Calibration Factor Fcp, We consider here the calibration factor Fca, relating the observed CH,O signal on the chart recorder to an absolute concentration. Although Fca,was one of the parameters gen- erated by the numerical fitting procedures, we also performed direct calibrations from time to time.To generate CH,O quantitatively,’ the reaction of methyl nitrite with fluorine atoms CH,ONO + F +CH,O + FNO (23) was employed. The LIF signal was a linear function of added [CH,ONO] so long as F atoms remained in substantial excess. In the linear regime, [CH,O] produced is proportion- al to [CH30NO) added,” and could be used to deduce Fcal after taking into account the experimentally measured losses of CH,O in the processes CH,O 4 products (21) CH,O + F -+ products (24) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Summary of modelling results pressure/Torr 2.54 2.54 2.54 2.73 2.56 2.98 3.19 2.81 2.41 2.87 2.37 2.35 3.51 3.41 3.46 3.42 2.58 2.61 [N0,]o/10'3 molecule cm-, 1.67 0.97 2.07 1.5 1.12 1.26 0.6 1.78 1.8 1.61 0.85 0.41 2.71 1.44 1.34 0.82 2.78 3.42 A[N03]/10'3 molecule cm-, 0.12 0.14 0.2 0.27 0.08 0.2 0.1 0.27 0.39 0.15 0.09 0.03 0.43 0.32 0.31 0.19 0.27 0.36 ratio, r 0.44 0.56 0.58 0.44 0.4 0.35 0.5 0.39 0.35 0.35 0.35 0.30 0.50 0.50 0.30 0.35 0.50 0.50 k,/10-'2 cm3 molecule-' s-' 1.01 1.29 1.33 1.01 0.92 0.81 1.15 0.90 0.81 0.81 0.81 0.69 1.15 1.15 0.69 0.81 1.15 1.15 r = 0.43 f0.09 and k, = (0.98 f0.20) x lo-', cm3 molecule-' s-', taking6 k,, = 2.3 x lo-', standard deviation. The values of ACNO,] are those determined experimentally.an3molecule-' s-'. Quoted errors are one Comparison with Results of Other Studies Only limited comparisons with data obtained by other workers are possible. Crowley et ~1.'~used a modulation technique to infer a rate constant for the reaction of CH,O, with NO, of 2.3 x lo-'' cm3 molecule-' s-', which is rather higher than, but not inconsistent with, the value we are suggesting here. However, after it was discovered that there was some non-photochemical production of NO, from HNO, , this figure was revised downwards by a factor of two to three.lg More recent experiments by the same group," in which CD302 and NO, were followed mass spectrometri- cally in a discharge-flow experiment, have suggested a rate coefficient of 2 x lo-', cm3 molecule-' s-', some ten times lower than the earlier published value.This value, obtained in a way that parallels (but does not exactly copy) our tech- nique, appears to us to be incompatible with the results that we have obtained by following the CH,O radical. We suggest that the key to the discrepancy may originate in the regener- ation of CH302 from CH,O, a process whose occurrence we have substantiated.6 Moortgat et d2'were unable to find evidence for this reaction. Our belief is that this failure to observe the reaction was a consequence of the manner in which the CD,O was generated (an upstream reaction between CD, and NO,). It seems quite likely that all the alkoxy radical could be consumed by reaction with NO,, so that none would be available for reaction with NO,.Since the alkoxy radical itself cannot be detected in the experiments of Moortgat et ~l.,~'these experiments cannot directly show, either, that CD30 is a product of the reaction of CD,O, with NO,. Regeneration of the peroxy radical could then make it appear that the reaction is much slower than it really is. We understand' ' that reanalysis using the scheme suggested by us here renders the earlier data of Moortgat et al." entirely compatible with our own measurements. Butkovskaya et ~1.~'have recently carried out discharge-flow experiments with LIF detection of CH,O. These experiments are almost exactly of the same type as our own, but seem to be limited to the CH,O + NO3 system.As far as we can judge, the data are very similar to ours.6 Analysis of the data using our sug- gested mechanism has enabled these workers to extract a rate coefficient for reaction (24in the same way that we did from the system in which CH,O was the initial reactant.6 The pr-- liminary value they obtain is 7 x lo-', cm3 molecule-' s-*. Our estimated value of k,, obtained in the same way is 1 x cm3 molecule-' s-', which is entirely compatible with the new result. However, as we explain in the previous paper,6 we believe that the rate coefficient is better measured in the way described here, using CH,O, as the starting radical. As it happens, the final result is numerically identical, but we regard both the interpretation and the calculation as being more firmly based in the second method adopted by us.Our conclusion is, then, that there is no real conflict in the experimental data obtained in a variety of systems, but only in the interpretation of what the results mean. Our systematic study of all the radical species involved (CH,, CH,O, CH,O, and NO,) in these complex chemical systems has given us a new understanding of the processes involved and allowed us to advance our interpretation of the results. It is interesting next to attempt a comparison of the reacti- vities of NO, and NO, towards the species H and CH,, OH and CH,O, and HO, and CH302. Table 3 summarises the rate parameters for these systems. Hydrogen atoms appear to react four times faster than methyl radicals do with either NO, and NO,.It would seem then that the mechanism is the same for the reaction of the species H and CH, with NO, and NO,, i.e. straightforward abstraction forming the alkoxy species, and it is tempting to invoke a simple steric argument to explain the difference in rate coefficient between H and Table 3 Summary of rate constants at room temperature for the reactions of R, RO and RO, (R reaction H + NO, -+ OH + NO, CH, + NO, -+ CH,O + NO, H + NO, + OH + NO CH, + NO, -,CH,O + NO OH + NO, -+ products CH,O + NO, -P products OH + NO, +products CH,O + NO, + products HO, + NO, -,products CH,02+ NO, + products HO, + NO, -,products CH,O, + NO, -+ products = H and CH,) with NO, and NO, k(298 K) /cm3 molecule -' s -' ref.1.1 x 10-'O 23 3.5 x lo-" 6 1.25 x lo-'' 24 2.3 x lo-'' 8 2.6 x lo-'' 4 2.3 x lo-', 6 2.4 x lo-'' lo" 2.1 x lo-" 8" 3.6 x lo-', 4 1.0 x this work 4.7 x lo-', 9" 6.5 x lo-', 9" " Denotes the high-pressure limit. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 CH, . These systems also suggest that there is little difference between the reactivity of NO, and NO, towards alkyl species. The similarity between NO, and NO, is less apparent when we consider their reactions with OH and CH,O. For the NO, case, formation of an adduct (HONO, and CH,ONO,) is the dominant reaction pathway, whereas for NO,, formation of a peroxy species (H02 and CH,O,) is the major pathway.Analogous 0-atom transfer with NO, would be endothermic. It may well be the case that the NO, reac-tions proceed through some energised complex,6 but that the stability of the peroxynitrate formed (HOONO, or CH,00N02) is insufficient for this channel to compete with decomposition. The reactions of CH302 and HO, with NO, lead to rela- tively unstable peroxy addition compounds. For reaction with NO,, the main channels yield three products, one of which is the oxy radical (CH,O or OH). One observation that can be offered about the chemical reactions of NO, with respect to the peroxy radicals, CH302 and HO, ,is that NO, appears almost to behave like the adduct NO-0,, and the processes lead to the same oxy radicals as are obtained with NO, although the rate constants for the processes involving NO, are somewhat smaller.Our preliminary indirect experiments,' with acylperoxy radicals, R * CO * 0, , suggest exactly the same type of behaviour. With the oxy radicals, CH,O and OH, the NO, radical acts, instead, as an oxidant, in a manner completely different from NO or NO,. Atmospberic Implications A conclusion that might be drawn from the considerations of the last paragraph is that NO, can behave in the night-time atmosphere simultaneously like OH and like NO behave during the day. The radical can abstract hydrogen atoms from many reactants and initiate oxidation processes, in the same manner as OH. The resulting organic radicals then add oxygen to form peroxy radicals, with which NO, interacts again, this time as though it were NO.In a quantitative discussion of night-time peroxy-radical chemistry, Platt et a/., point out the importance of NO,- initiated oxidation. These workers invoke the reaction between CH302 and NO, as a step in a route that could yield significant concentrations of OH radicals in the tropo- sphere at night. The central reactions of the scheme as it involves CH,O and CH302 are CH302+ CH302--* CH,O + CH,O + 0,; 30% (9) CH,O + 0, -+ HO, + HCHO (5) HO, + NO, + OH + NO, + 0,; 80% (6) CH,O, + NO, +CH,O + NO, + 0, (1) Reactions (9) and (6) are particular channels in which more than one branch is known, and the percentages indicated are fairly well established for these channels.In particular, the remaining 70% of the self reaction of CH,O, does not lead to any radicals, so that the reaction is generally terminating. If the reaction between CH,O, and NO, is both rapid and leads virtually exclusively to the products shown in channel (l),then the possibility arises that CH302 can be intercepted by NO, and the subsequent steps generate OH through process (6), again involving NO,. Platt et al., calculate that [OH] could approach 10' molecule ern-,, and that the rate of its production could be only one order of magnitude less by night than it is by day. This conclusion depends on the rate coeficient for reaction (1). The rate of channel (1) as a 1209 source of HO, ,and hence as a source of OH, will exceed that of channel (9) when ~NO3l/[cH302la 2kdk1 For" k, = 1.1 x lop1, cm3 molecule-' s-' and'8 k, = 2.3 x lo-', cm3 molecule-' s-', the inequality becomes [NO,]/[CH,O,] 2 0.1.This condition appears to have been met during the measurement campaigns in Brittany in 1989 but not in 1988.26With the lowest values of k, compatible with our measurements (say 0.5 x lo-', cm3 molecule-' s-'j, the corresponding limiting ratio of concentrations would be about 0.4, a condition possibly met only at the highest NO, encountered. However, exceptionally high [CH,O,] concentrations were inferred from the measure-ments. Concentrations of NO, also seemed abnormally low. Similar observations have been made recently at Schauins- land in the Black Forest.27 In these latter measurements, there is a clear anti-correlation between concentrations of NO, and RO, radicals, a result strongly suggestive of an interaction between the two radical species.Taking a 'normal' night-time concentration of NO, of lo9 molecule an-,, the concentration of CH302 would only have to be less than about 3 x lo9 molecule cm-, for reaction (1) to dominate over reaction (9), even using the lower limit of our rate constant. This limitation seems hardly likely to be restrictive. The essential condition for reaction (1) to be the major source of OH therefore is likely to be met in continen- tal and especially in urban areas. For reaction (1) to be a significant source of OH at night is another matter, because it is now necessary that [CH,02] is relatively high also.The maximum rate of OH production through reactions (1) and (6), taking k, = 1.5 x lo-', cm3 molecule-' s-', and a branching ratio4 of 80% for channel (6), is 0.8 x k, x [CH,O,][NO,]. For [NO,] = lo9 molecule cmP3 and [CH,O,] = lo7 molecule cm-,, our results suggest that this rate would be less than 1.2 x lo4 molecule cmP3 s-l, which is more than 100 times lower than the day-time rate of OH production. The recent measurements from the Black Forest,' give [NO,] = 8 x lo7 molecule cm-, when [RO,] = 9 x lo* cm3 molecule-' s-l (at 0300 CET). For these conditions, the maximum production rate of OH is thus about 9 x lo4 molecule an-,s-l if all RO, radicals possess a reactivity similar to CH,O, .Although this production rate is still less than 10% of that during the day, it seems that the possibility of the reaction playing some r61e in 'normal' atmospheric chemistry at night should not be discounted, and it clearly could be important when anomalous concen- tration conditions are present such as those found in the Brit- tany campaign. We would like to express our gratitude to the NERC (grant GR3/7359) for support for this project, and to the CEC and the Institute Frangais du Petrole, under whose auspices various parts of this and related work were carried out. We thank Richenda Connell for preparing the methyl nitrite samples used in this work. D.E.S. would like to thank the SERC for a research studentship during the tenure of which this work was conducted.References 1 P. D. Lightfoot, R. A. Cox, J. N. Crowley, M. Destriau, G. D. Hayman, M. E. Jenkin, G. K. Moortgat and F. Zabel, Atmos. Enuiron., A, 1992,26, 1805. 2 U. Platt, G. Le Bras, G. Poulet, J. P. Burrows and G. K. Moor-tgat, Nature (London),1990,348, 147. 3 I. W. Hall, R. P. Wayne, R. A. Cox, M. E. Jenkin and G. D. Hayman, J. Phys. Chem., 1988,92,5049. 1210 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 4 A. Mellouki, G. Le Bras and G. Poulet, J. Phys. Chem., 1988,92, 2229. 18 J. N. Crowley, J. P. Burrows, G. K. Moortgat, G. Poulet and G. Le Bras, Znt. J. Chem. Kinet., 1990,22,673. 5 6 7 8 9 10 A. R. Ravishankara, Annu. Rev. Phys. Chem., 1988,39,367. P.Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross and R. P. Wayne, J. Chem. SOC., Faraday Trans,, 1994,90,1197. A. R. Curtis and W. P. Sweetenham, FACSIMILE, AERE- R12805, Harwell Laboratory, Oxfordshire, UK, 1988. P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, A. D. Parr, D. E. Shallcross, R. P. Wayne and F. Caralp, J. Chem. SOC., Faraday Trans., 1993,89,4163. R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson, J. A. Kerr and J. Troe, J. Phys. Chem. Ref: Data, 1989,18,881. W. B. DeMore, S. P. Sander, D. M. Golden, M. J. Molina, R. F. Hampson, M. J. Kurylo, C. J. Howard and A. R. Ravishankara, Chemical Kinetics and Photochemical Data for use in Strato- spheric Modeling. Evaluation Number 10, JPL Publication 92-20, 1992. 19 20 21 22 23 24 J. P. Burrows, J.Crowley and D. Maric, EUROTRAC Annual Report 1991, Part 8 (LACTOZ),EUROTRAC ISS, Garmisch- Partenkirchen, 1992, p. 81. G. K. Moortgat, J. Crowley, F. Helleis, 0. Hone, W. Raber and C. Zahn, EUROTRAC Annual Report 1992, Part 8 (LACTOZ), EUROTRAC ISS, Garmisch-Partenkirchen, 1993, p. 149. J. N. Crowley, persoKa1 communication N. Butkovskaya, V. Daele, D. Johnstone, G. Laverdet, G. Poulet and G. Le Bras, EUROTRAC Annual Report 1992, Part 8 (LACTOZ), EUROTRAC ISS, Garmisch-Partenkirchen, 1993, p. 127. R. B. Boodaghians, C. E. Canosa-Mas, P. J. Carpenter and R. P. Wayne, J. Chem. SOC., Faraday Trans., 1988,84,931. M. A. A. Clyne and P. B. Monkhouse, J. Chem. Soc., Faraday Trans. 2, 1977,73,298. 11 12 13 F. G. Simon, W. Schneider and G. K. Moortgat, Znt. J. Chem. Kinet., 1990,22, 791. N. L. Arthur, J. Chem. SOC., Faraday Trans., 1986,82,331. W. Tsang and R. F. Hampson, J. Phys. Chem. Ref: Data, 1986, 15, 1087. 25 26 27 P. S. Owen, D. Phil Thesis, University of Oxford, 1994. T. Brauers, H. P. Dorn and U. Platt, in Physico-chemical Behav- iour of Atmospheric Pollutants, ed. G. Restelli and G. Angelletti, Kluwer, Dordrecht, 1990. D. Mihelcic, D. Klemp, P. Musgen, H. W. Patz and A. Volz- 14 J. Heicklen, Adv. Photochem., 1988, 14, 177. Thomas, J. Atmos. Chem., 1993,16,313. 15 U. Meier, H. H. Grootheer, G. Riekert and Th. Just, Ber. Bun- 16 senges. Phys. Chem., 1985,89,325. M. J. Frost and I. W. M. Smith, J. Chem. Soc., Faraday Trans., 1990,86 1757. 17 P. Biggs, C. E. Canosa-Mas, J-M. Fracheboud, D. E. Shallcross and R. P. Wayne, in preparation. Paper 3/06980J; Received 23rd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001205

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1211-1215 121 1 Molecular Conformations and Rotation Barriers of 2-Halogenoethanethiols and 2-Halogenoethanols :An arb initio Study Giuseppe Buemi Dipartimento di Scienze Chimiche, Universita di Catania, Viale A. Doria nr. 6,95125,Catania, Italy The molecular geometries of all the possible conformations of 2-halogenoethanethiols and 2-halogenoethanols (F, CI, Br, I) have been fully optimized at the ab initio MP2/6-31G** level (for Br and I the LANLlDZ basis was adopted). Whilst for all 2-halogenoethanols and for 2-fluoroethanethiol the Gg' structure was found to be the most stable, the Tg rotamer is favoured for 2-chloro-, 2-bromo- and 2-iodo-ethanethiols. In these latter molecules the hydrogen-bond strength seems to be insufficient to overcome the non-bonded repulsive interactions and to stabilize the Gg' conformer.The rotation barriers around the C(l)-C(2) bond are rather high, whilst those concerning the SH and OH groups are generally small and of similar size. Experimental measurements (ref. 1-9 and references cited therein) and ab initio calculations performed by using various basis sets (ref. 10-12 and references cited therein) have shown that a gauche-gauche (Gg) conformation is the most stable rotamer of halogenoethanols, in both the gas and liquid phases. This stability is usually attributed to formation of a Y. .H-0 intramolecular hydrogen bridge (which cannot occur in the trans structures) although doubts about this interpretation arise from the prevalence of the Gg rotamer in 2-fluoroethylacetate and 2-fluoroethyltrichloroacetate,where no hydrogen bond is po~sible.'~*'~ The related hydrogen- bond energy, if any, is expected to be reasonably small.In 2-fluoroethanol it was evaluated to be 9.2 and 7.95 kJ mol- ' from electron diffraction2' and ab initio calculations,' respectively, with negligible contribution from the gauche effect.I2 A strength of ca. 10 kJ mol-' is suggested for 2- chloroethanol,2b in contrast with a previous study," where it was estimated to be ca. 2.7 kJ mol-' lower than in 2- fluoroethanol. Since the strength of a hydrogen bridge depends highly on the electronegativity of the atoms involved, its energy in 2-halogenoethanethiols must be lower than in 2-halogenoethanols, so that in the sulfurated mol- ecules the Tt and/or the Tg conformations would be pre- ferred.Experimental data on these latter compounds are scarce and not conclusive. The microwave spectrum of 2-chloroethanethiol, although largely intractable,' allowed identification of only one rota- tional isomer, which exists in a heavy atoms trans conforma-tion, but also showed numerous lines arising, probably, from gauche species. IR spectral6 suggest that the gauche con-former is ca. 0.5 kcal mol-' less stable than the trans con-former. Ab initio calculations (3-21G and 6-31G*)17*18 indicate that the most stable rotamer is the trans-gauche structure, which was found to be 0.79 and 1.32 kcal mol- more stable than the Gg' and Tt conformers,? respectively.More recent 3-21G cal~ulations'~ confirm the gauche accommodation of the S-H group in both 2-fluoro- and 2-chloro-ethanethiol. No theoretical study was found in the literature for 2-bromo- and 2-iodo-e t hanet hiols. The aim of the present paper is a systematic ab initio study of the title compounds in order to calculate geometries and energies of all the possible conformations of 2-fluoro-, 2-chloro-, 2-bromo- and 2-iodo-ethanethiol, and to estimate roughly the hydrogen-bridge energy if any (or at least, the t The Gg and Gg' rotamer differ from each other by having differ- ent torsion angles around the C-C bond (in the former ca. 60°, in the latter ca. -60"). existence of favourable accommodation for hydrogen-bridge formation). A further goal is to evaluate the rotation barriers in the various interconversion pathways of the most stable rotamers.For comparison purposes, calculations were carried out also on the corresponding oxygenated com-pounds, 2-halogenoethanols, for which more experimental and theoretical data are available in the literature. Calculations All calculations were carried out by means of the GAUSSIAN92 program,20 running on a DIGITAL ALPHA- 3400 workstation as a translated image of VAX executable sources. For building the energy curves for 2-fluoro- and 2-chloro-ethanethiols the 3-21G* basis set, with fully geometry optimization, was adopted. Then all the minima and maxima points were fully optimized at the MP2/6-31G** level.Since such bases are not available for bromine and iodine, calcu- lations on the 2-bromo- and 2-iodo-derivatives were per- formed using the LANLlDZ basis set," which uses the Dunning-Huzinaga valence double zeta (D95V) basis22 for the first-row elements and the Los Alamos ECP +DZ basis23 (ECP = effective core potentials) for the elements from Na to Bi. Here also, minima and maxima were then reoptimized taking into account the correlation energy (MP2/ LANLlDZ). Fig. 1 and 2 were produced by means of the Harvard Graphics program, version 2.0. Results and Discussion The energy curves of 2-halogenoethanethiols, calculated at the 3-21G* level for rotation from 0" to 360" around the C-C and C-S bonds are shown in Fig. 1. Those of the corresponding 2-halogenoethanols were assumed to have analogous trends.Such an assumption is supported by the energy curves for 2-fluoro- and 2-chloro-ethanols, shown in Fig. 2, which were calculated at both the MP2/3-21G* and the MP2/LANLlDZ level (with fully geometry optimization) in order to check the limit of comparability of the results from the two bases. The optimized geometrical parameters obtained by MP2/LANLlDZ calculations are not reported here, but are available upon request. The main differences from those of the MP2/3-21G* basis are: (i) The energy curves concerning the rotation around the C-C bond show analogous trends whilst differences are noted in those concerning the rotation of the OH group. In particular the Gt conformation is no longer found when the LANLlDZ basis is adopted (a flex point appears instead of a minimum-energy point); optimization of the geometry of 1212 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 such a rotamer evolves towards the Gg' conformer. This does not occur for 2-bromo- or 2-iodo-ethanol or any of the 2- --halogenoethanethiols. ;20 g 20 (ii) The bond lengths predicted by the LANLlDZ basis are 7 7 longer than those from the 3-21G* and 6-31G** (with and $1 0 $10 without inclusion of correlation energy); in some cases the d 0 d 0 difference can reach ca. 0.1 A. Bond angles are much less affected, differences being limited to one degree or less. 0 60 180 -60 0 0 60 180 -60 0 120 -120 120 -120 (iii) Inversion of the stability order can occur when AE values are very small.In the light of the above remarks we must remember that in the following discussion only qualitative comparison can be made between the results for bromo- and iodo-derivatives I-(MP2/LANLlDZ) and those for fluoro-and chloro-z 20 derivatives (MP2/6-3 lG**). 7 From Fig. 1 it can be seen that rotation around C-C $10 gives rise to three minima, whilst rotation around C-S determines two other minima, so that five rotamers are pos- n" 0 60 180 -60 0 0 60 180 -60 0 sible for each molecule. Following ref. 10, they are labelled as 120 -120 120 -120 Gg, Tg, Gg', Gt and Tt (Fig. 3), where the upper-case letter torsion ang le/deg rees torsion angle/degrees refers to the torsion angle around the C-C bond and the Fig.1 Potential-energy curves calculated at 3-21G* (F and C- lower-case letter to the torsion angle around the C-X bondderivatives) and LANLlDZ (Br and I derivatives) levels. (0)Series 1 : rotation around the C-C bond; (+) series 2: rotation around the (rotation of the SH or OH groups). All except the Tt form, C-X bond in the Gg structure; (*) series 3: rotations around the have statistical weight, g = 2. C-X bond in the Tg structure. (a) 2-Fluoroethanethiol, (b) 2-The main geometrical parameters of the possible rotamers chloroethanethiol, (c) 2-bromoethanethiol, (d) 2-iodoethanethiol. of 2-halogenoethanethiols are reported in Table 1; those of Rotation around the C-X bond in the Gg' isomer produces a curve 2-halogenoethanols are omitted to save space whilst the sta- symmetrical with that of the Gg rotamer.On going from left to right, bility order and dipole moments are summarized in Table 2. the three minima correspond to Gg, Tg, G'g (series l), Gg, Gt, Gg' (series 2), Tg, Tt and Tg' (series 3) structures. Note that G'g and Tg However, the complete optimized geome!ries of all the title are equivalent to the Gg' and Tg' conformers, respectively. compounds are available upon request. A first analysis of Table 1 Calculated and experimental geometries of 2-halogenoethanethiols (distances in A, angles in degrees, energies in kJ mol-l) 2-fluoroethanethiol" 2-chloroethanethiol" 1.513 1.510 1.513 1.514 1.510 1.518 1.516 1.515 1.515 1.515'C( 1) -C(2) 1.816 1.816 1.816 1.822 1.823 1.815 1.815 1.817 1.825 1.824'C(2)-S(3) 'C(l)-Y 1.389 1.398 1.396 1.395 1.395 1.780 1.786 1.787 1.783 1.783 'S(3)-H(5) 1.332 1.331 1.332 1.330 1.331 1.332 1.331 1.332 1.331 1.331 'C(2) -H 1.089 1.089 1.088 1.088 1.083 1.089 1.091 1.088 1.088 1.089 'C(1) -H 1.092 1.090 1.090 1.090 1.090 1.090 1.089 1.086 1.087 1.087 'S(3)-Y 3.548 3.109 4.003 3.967 2.993 3.478 3.490 4.386 4.347 3.352 'H(5) -Y 3.150 2.484 4.199 4.718 4.182 3.995 2.877 4.576 5.076 4.542 6Y-C(U-C(2) 110.2 109.1 108.7 108.3 109.3 112.7 113.1 110.6 110.4 112.3 6C(1)-C(2)-s 114.4 112.8 112.2 108.8 109.6 115.5 114.9 111.8 108.0 111.0 'C(2) -S-H(5) 95.8 94.7 96.1 95.8 95.8 95.7 95.9 96.0 95.7 95.3 my-c-c-s 61.8 -63.0 178.6 179.2 58.1 66.0 -68.7 178.2 179.9 63.8 wH(5) -S-C -C 64.0 54.1 67.8 175.9 152.8 68.7 63.8 67.9 179.0 156.1 AE 8.14 0.00 3.01 9.72 8.88 8.51 1.81 0.00 6.53 10.69 P 3.06 2.16 1.10 1.20 3.41 3.15 2.05 1.25 1.37 3.44 w(%) 3 72 22 1 2 2 31 64 2 1 2-br~moethanethiol~ 2-i~doethanethiol~ 'C( 1 ) -C(2) 1.546 1.545 1.542 1.542 1.543 1.550 1.549 1.544 1.545 1.547 'C(2) -S(3) 1.904 1.903 1.912 1.92 1 1.915 1.903 1.903 1.913 1.922 1.914 'C(1) -Y 2.032 2.040 2.046 2.040 2.037 2.200 2.206 2.210 2.205 2.204 'S(3)-H(5) 1.374 1.372 1.374 1.373 1.374 1.374 1.372 1.374 1.374 1.374 'C(2) -H 1.103 1.103 1.101 1.101 1.103 1.101 1.103 1.101 1.101 1.103 'C(1)-H 1.101 1.100 1.099 1.099 1.100 1.090 1.101 1.100 1.100 1.100 'S(3) -Y 3.734 3.761 4.724 4.687 3.620 3.865 3.924 4.893 4.857 3.779 'H(5)-Y 4.304 3.1 16 4.95 1 5.424 4.855 4.101 3.265 5.112 5.587 4.984 6, -C(l)-C,2) 112.6 112.0 110.0 109.8 112.4 113.6 113.1 110.8 110.5 113.6 dC(1)-C(2)-s 115.2 114.7 111.1 107.5 110.8 115.3 115.0 11 1.2 107.6 110.8 'C(2) -S-H(5) 96.2 96.7 96.6 96.3 95.9 96.2 96.9 96.5 96.4 96.0 "Y-c-c-s 68.3 -71.7 178.2 180.0 66.8 67.9 -72.9 177.8 180.0 68.5 OH(5)-S -C -C 68.5 66.0 70.7 180.0 155.8 66.1 67.8 69.4 180.0 164.2 AE 9.58 2.90 0.00 6.05 12.48 10.04 5.10 0.00 6.31 13.51 cc 3.77 2.35 1.48 1.63 4.11 3.27 2.02 1.17 1.38 3.55 w(%) 2 23 72 3 2 11 84 3 " MP2/6-31G** basis.MP2/LANLlDZ basis. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 c 1.20z 3 $10 0 I Ii II I I I I 0 60 120 180 -120 -60 0 torsion angle/degrees I JI I I I I 0 60 120 180 -120 -60 0 torsion angle/degrees Fig.2 Energy curves for (a)2-fluoroethanol and (b)2-chloroethanol calculated at MP2/3-21GS (series 1 and 2) and MPYLANLIDZ (series 3 and 4). Series 1 and 3: rotation around the C-C bond. Series 2 and 4: rotation around the C-X bond in the Gg structure. On going from left to right, the three minima correspond to Gg, Tg, Gg (series 1 and 3), Gg, Gt and Gg' (series 2 and 4). Gg and Gg' are equivalent conformers. such geometries evidences that the C-Y (Y = halogen) bond length is practically constant in the sulfurated as well as in the oxygenated compounds. The C-S-H angle ranges from 95" to 97", independent of the basis set used for the calcu- lations, whilst the C-0-H angle undergoes more impor- tant changes (at least for the most stable conformer) on passing from the less to the most bulky halogen.Calculated and experimental data agree rather well for 2-fluoro- and 2- chloro-ethanols; the agreement is poor for 2-bromoethanol, but we must bear in mind that several geometrical param- eters in the cited microwave studies are assumed values and that the LANLlDZ basis overestimates bond lengths. Gt Tg Tt Fig. 3 Possible conformers of 2-halogenoethanethiols (Y = halogen, X = S). The Gg.' rotamer is similar to the Gg rotamer, but MY-C-C-X) is near to -60". The stability order found for the various rotamers of 2- fluoro- and 2-chloroethanols is analogous to that reported by Murto et al.," but 6-31G** and MP2 energies reported there are different from ours because of the partial geometry opti- mization performed by those authors.Use of a triple-zeta basis set augmented by two sets of polarization functions on C, F and 0, and one set of polarization functions on the H atoms12 produces AE values lower than those obtained in the present paper for 2-fluoroethanol. Our results indicate that the Gg' is always the most stable and the Gg the less stable conformation of 2-halogenoethanols. According to the Boltz- mann equation, and in agreement with experimental find- ings, the Gg' is therefore the prevailing rotamer at room temperature; its percentage decreases on increasing the size of the halogen atom, so that in 2-iodoethanol an equilibrium between the Gg' and Tg conformers (56% and 31%, respectively) is predicted. It is noteworthy that AE between the Gg' and Tg forms of 2-fluoroethanol (10.83 kJ mol-') is in excellent agreement with the value of 11.3 kJ mol-' esti-mated from electron diffraction studies.2b Electron diffraction measurements suggest also that the energy difference between the gauche (our Gg') and trans (our Tg) forms of 2-chloroethanol is 10.04 kJ mol-'.Our calcu- lations predict a value of 6.31 kJ mol-', which is lower than that figure, but very close to the value of 5.02 kJ mol-', esti-Table 2 Calculated stability order, dipole moments and percentages of 2-halogenoethanol conformations (energies in kJ mol-', p in D") Gg Gg' Tg Tt Gt Gg Gg' Tg Tt Gt 2-flu~roethanol~ 2-chl~roethanol~ AE cc w (Yo) 13.63 3.33 - 0.00 1.89 97 10.83 1.79 1 10.80 2.03 1 12.46 3.15 1 10.90 3.44 1 0.00 1.73 88 6.3 1 1.94 7 7.16 2.29 3 10.37 3.35 1 2-bromoethanol' 2-iodoethanol' AE cc w (Yo) 11.58 4.11 1 0.00 2.13 76 4.24 2.20 14 4.09 2.63 7 10.02 3.89 1 9.01 3.89 2 0.00 1.87 56 1.51 1.95 31 2.83 2.37 2 8.1 1 3.65 9 1 D x 3.33564 x lo-'' C m.MP2/6-31G** basis. MP2/LANLlDZ basis. mated from the relative intensities of the C-Cl stretching bands of the gauche and trans isomers in the vapour phase, measured as a function of the temperature up to 165"C.7 Acceptable agreement is found also for the corresponding AE for 2-bromoethanol since the gauche-trans enthalpy differ- ence, evaluated by the same technique is 6.07 kJ mol-', whilst our results give 4.24 kJ mo1-'.The agreement improves if comparison is made with the AE of 5.44 kJ mol-' cited in ref. 5(b). The Tg and Tt rotamers are suggested by calculation to be nearly isoenergetic isomers. The situation with the 2-halogenoethanethiols is rather dif- ferent than with the 2-halogenoethanols. In fact, the Gg' is predicted to be the most stable rotamer only for the 2- fluoroderivative whilst the Tg form is the most stable confor- mation for 2-chloro-, 2-bromo- and 2-iodo-ethanethiols ;its percentage increases on passing from C1 (64%) to Br (72%) and I (84%), i.e. on increasing the size of halogen atom (and on decreasing its electronegativity). As far as geometrical parameters are concerned, bond lengths and bond angles of the left-side moiety undergo negli- gible variations on passing from halogenoethanols to halogeno- ethanethiols. The halogenoethanol rotamers are always more polar than the corresponding halogenoethanethiols; in any case the Gg and Gt forms show the largest dipole moment values in both series of compounds.Tg and Tt are generally the less polar halogenoethanethiol conformers. The theoretical results for 2-chloroethanethiol agree very well with the experimental findings from microwave spectros- copy.15 The energy difference between the Tg and Gg' con-formers is 1.81 kJ mol-', to be compared with the 2.09 kJ mol-suggested by IR studies.16 The stability order is analo- gous to that obtained at the 3-21GI7 and 6-31G* levels," but the energy differences among the various conformers decreases when correlation is considered.Hydrogen Bonding In the Gg' conformations of 2-halogenoethanols and 2-halogenoethanethiols, the distances between the halogen and oxygen (or sulfur) atoms and between the halogen and H(5) atoms are constantly lower than the sum of the related Van der Waals radii, so that the formation of a hydrogen bridge is conceivable. This fact has been invoked to justify the greater stability of the Gg' isomer with respect to the other rotamers of 2-halogenoethanols and it could also determine the stabil- ity order of 2-halogenoethanethiols. Calculations show that when oxygen is substituted with sulfur the hydrogen-bond strength decreases and, consequently, the Gg' structure is destabilized.Bearing in mind the electronegativity of halo- gens, we must expect that the strength of the hydrogen bridge weakens on going from F to I, and becomes no longer suffi- cient to overcome the repulsive interactions. Indeed, no evidence of intramolecular hydrogen bonding was found in b-chloro- and b-bromo-ethylmercaptans when IR measurements, with different solvents, were made for the S-H stretching modes.16 On the other hand, a more careful analysis of geometrical data evidences that the S.* .Yand *0..Y(Y = halogen) distances are lower than the sum of the van der Waals radii also in the Gg and Gt structures, i.e. also when the Y..aH(5) distance is not suitable for hydrogen-bond formation. This could mean that the total energies of such rotamers could be affected by interactions between the sulfur (or oxygen) and the halogen atoms similar to those between J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Hydrogen-bond strength (kJmol-') of the title compounds evaluated with respect to the various rotamers assumed to be hydrogen bond free 2-Y-ethanethiols 3.01F 8.88 8.14 C1 8.88 6.70 -1.81 Br 9.58 6.68 -2.90 I 8.41 4.94 -5.10 2-Y-ethanoIs F 12.46 13.63 10.83 C1 10.37 10.90 6.3 1 Br 10.02 11.58 4.24 I 8.11 9.01 1.51 9.72 4.72 4.24 1.21 10.80 7.16 4.09 2.83 between the energy of the examined isomer and the energy of another conformation where no hydrogen bridge is present. The obtained figures are not absolute values because they are more or less affected by the different geometries of the hydrogen-bonded structure and of the minimum-energy refer- ence structure assumed to be hydrogen bond free.In the present case we could select each of the reported conformers, except the Gg' one, as hydrogen bond free reference structure. Inspection of the results obtained (see Table 3) evidences that E,, for 2-fluoroethanol ranges from 10.80 to 13.6 kJ mol-'. The lower datum agrees well with the values of 9.20 and 7.95 kJ mol- ' suggested by the most recent electron diffractionZb and ab initid2 investigations, respectively. Moreover, EHB of 2-substituted ethanethiols is always lower than that of the corresponding 2-substituted ethanols, and decreases on going from F to I, except in the ethanethiol series when it is calcu- lated with respect to the Gt rotamer (in this case equal strengths are found for 2-fluoro- and 2-chloro-ethanethiols).This lowering of EHB is in line with the lower electronega- tivity of sulfur with respect to oxygen and could justify the inversion of the Gg' and Tg stability order found on passing from 2-fluoro- to 2-chloro-ethanethiol. Since EHB for F and C1 derivatives comes from a different basis set from that adopted for Br and I derivatives, compari- son between these results and the previous values may be meaningless. Rotation Barriers The high flexibility of the compounds studied allows several interconversions between two or more conformations by rotation around the C-C and/or C-X bonds.The rotation barriers to be overcome in such pathways are reported in Table 4. The Tg e Gg' interconversion pathway implies rota- tion around the C(l)-C(2) bond which shows its maximum energy value (see Fig. 1) is at ca. -120". Starting from the Gg' conformer it is possible to reach the Gt and Gg rotamers by rotation of the SH or OH groups. For the Gg'e Gt pathway the maximum energy point (3- 21G* calculations) lies near -135" for F and C1 derivatives and near -150" for Br and I derivatives (the energies at -135" and -150" are not appreciably different in each compound). For the GteGg pathway the maximum is centred at 120". Rotation of the SH or OH group allows interconversion between the Tg and Tt conformations, cross- sulfur and oxygen noted by Kucsman and co-~orkers.~~-~~ ing through an energy maximum centred at 135" for all com- To understand better the reason for the different order of pounds.As can be seen from data reported in Table 4, the SH stability of 2-halogenoethanols and 2-halogenoethanethiols and OH rotation barriers are of the same order in both series we need a quantitative evaluation of the hydrogen-bond of compounds. The highest values are those of the Gg' e Gt energy (EHB), which is usually assumed to be the difference interconversion pathway; most of the remaining ones are J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Barriers (kJ mol-') in some interconversion pathways 2-halogenoethanethiols 2-halogenoethanols F C1 Br I F CI Br I Gg'+ Tg 21.84 19.09 16.95 16.10 24.37 23.75 19.82 19.59 Tg + Gg' Gg + Gt Gt + Gg' Gg' + Gt Gt + Gg Tg + Tt Tt -+ Tg 18.83 2.32 4.2 1 13.10 1.58 6.72 0.01 20.90 3.15 3.19 12.07 0.97 6.86 0.33 19.85 3.21 1.77 11.34 0.32 6.12 0.07 21.20 4.03 1.37 9.78 0.55 6.48 0.17 13.54 4.91 1.13 13.58 4.9 1 2.87 2.91 17.43 5.75 0.44 10.80 6.28 3.48 2.63 15.58 5.26 0.07 10.10 6.82 2.20 2.35 18.08 5.48 0.30 8.42 6.38 3.37 2.05 extremely small so that the related interconversions may occur easily at room temperature, especially from the less to the most stable rotamers.On the whole, the 2-halogenoethanol barriers are in line with those reported in ref. 10. 5 6 7 (a) A. Almenningen, 0.Bastiansen, L. Fernholt and K.Edberg, Acta Chem. Scand., 1971,25, 1946; (b) A. Almenningen, L. Fern-holt and K. Kveseth, Acta Chem. Scand., 1977,31,297. J. Pourcin, G. Davidovics, H. Bodot, L. Abouaf-Marguin and B. Gauthier-Roy, Chem. Phys. Lett., 1980,74, 147. P. Buckley, P. A. Giguere and M. Schneider, Can. J. Chem., Conclusions 8 1969,47,901. P. Buckley, P. A. Giguere and D. Yamamoto, Can. J. Chem., The present study has shown that 2-halogenoethanethiols, analogously to 2-halogenoethanols, exist mainly in the Gg' and Tg conformations. At room temperature, the former rotamer prevails in the oxygenated molecules and in 2-fluoroethanethiol, whilst the latter is the most stable one in the remaining sulfurated compounds. In Br and I derivatives an equilibrium with remarkable percentages of the two struc- tures is predicted; however, their interconversion should be 9 10 11 12 13 1968,46,2917.L. Radom, W. A. Lathan, W. J. Here and J. A. Pople, J. Am. Chem. SOC.,1973,95,693. J. Murto, M. Rasanen, A. Aspiala and L. Homanen, J. Mol. Struct. (Theochem), 1983,92,45. J. Murto, M. Rasanen, A. Aspiala and T. Lotta, J. Mol. Struct. (Theochem),1984,108,99. D. A. Dixon and B. E. Smart, J. Phys. Chem., 1991,95,1609. R. C. Griflith and J. D. Roberts, Tetrahedron Lett., 1974, 39, 3499. difficult owing to the relatively high barriers to be overcome. Lower barriers are found for the SH and OH groups, which in some cases can undergo free rotation very easily. The different stabilities of the Gg' and Tg rotamers in the oxygenated and sulfurated molecules can be attributed to the different strength of the hydrogen bridge present in the 14 15 16 17 R.J. Abraham and J. R. Monasterios, Org. Magn. Reson., 1973, 5, 305. R. N. Nandi, M. F. Boland and M. D. Harmony, J. Mol. Spec- trosc., 1982,92,419. M. Hayashi, Y. Shiro, M. Murakomi and H. Murata, Bull. Chem. SOC.Jpn., 1965,38,1740. M. Osaku, J. Mol. Struct. (Theochem), 1986,138,283. former structure, owing to the lower electronegativity of sulfur with respect to oxygen. Although it is not possible to obtain absolute energy values for the bridge strength because it is highly dependent on the conformation assumed to be hydrogen bond free, the numerical values here obtained are in line with the previous justification. Moreover, since elec- tronegativity (as well as the hydrogen-bond energy) decreases on going from F to I, in both series of molecules the percent- age of the Tg rotamer increases on increasing the size of the halogen atom.Financial contribution from the Italian Minister0 dell'universita e della Ricerca Scientifica e Tecnologica (MURST),Roma, is gratefully acknowledged. 18 19 20 21 22 23 24 R. Benassi and F. Taddei, J. Mol. Struct., 1990,205, 177. S. L. Emery, G. R. Famini, J. 0. Jensen, J. M. Leonard and D. J. Reutter, Phosphorus, Sulfur Silicon Relat. Elem., 1990, 53, 373. Gaussian 92, Revision B, M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gom- perts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzales, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart and J. A. Pople, Gaussian Inc., Pittsburgh, PA, 1992. M. Frisch, J. Foresman and A. Frisch, Gaussian 92 User's Guide. T. H. Dunning and P. J. Hay, in Modern Theoretical Chemistry, Plenum, New York,1976, ch. 1, pp. 1-28. (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b) W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985,82,284; (c)P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985,82,299. J. G. Angyan, R. A. Poirier, A. Kucsman and I. G. Csizmadia, J. References 25 Am. Chem. SOC.,1987,109,2237. A. Kucsman, I. Kapovits, M. Czugler, L. Parkanyi and A. K. S. Buckton and R. G. Azrak, J. Chem. Phys., 1970,52,5652. (a)K. Hagen and K. Hedberg, J. Am. Chem. SOC.,1973,95,8263; (b) J. Huang and K. Hedberg, J. Am. Chem. SOC.,1989,111,6909. 26 Kalman, J. Mol. Struct., 1989, 198, 339. L. Parkanyi, A. Kalman, A. Kucsman and I. Kapovits, J. Mol. Struct., 1989,198, 339. R. G. Azrak and E. B. Wilson, J. Chem. Phys., 1970,52,5299. K. G. R. Pachler and P. L. Wessels, J. Mol. Struct., 1970, 6, 5299. Paper 3/06863C ; Received 16th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001211

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1217-1221 Thermophysical Properties of Liquid m-Xylene at High Pressures Mercedes Taravillo, Susana Castro, Valenth Garcia Baonza," Mercedes Caceres and Javier Nuiiez Departamento de Quimica Fkica ,Facultad de Ciencias Quimicas, Universidad Complutense de Madrid, 28040-Madrid, Spain Experimental ppT measurements of rn-xylene obtained with an expansion technique are reported from 230 to 298 K and up to 110 MPa, or freezing pressure when lower. The experimental results have been correlated in terms of a recently proposed equation of state which has been used to calculate the isothermal compressibility, ,K~ and the thermal expansion coefficient, cc,, as functions of pressure and temperature. Experimental density results for liquid toluene, which was used to calibrate our experimental device, are also reported for the iso- therms of 223 and 303 K.The ability of simple power functions recently proposed to correlate high-pressure results has been tested. The extrapolation capabilities of these functions have been confirmed using experimen- tal results from the literature for the two compounds studied. Very recently we have reported experimental pp T measure-ments and derived thermodynamic properties of mesitylene obtained with an expansion technique.' We also studied the ability of simple power functions to represent the experimen- tal measurements as well as the possibility of using those functions for extrapolation. In this work we present an analogous study for rn-xylene for temperatures between 230 and 298 K and pressures up to 110 MPa.Experimental measurements for toluene, a com- pound which was used to calibrate the experimental device, are also given for the temperatures 223 and 303 K. The experimental results have been correlated using a recently derived equation of state (EOS) which has been used ,to calculate the isothermal compressibility, K~ and the thermal expansion coefficient, a,, of rn-xylene as functions of pressure and temperature. These quantities have been com- pared with data taken from literature up to 200 MPa. Besides the importance of the experimental results them- selves, which cover a wide ppT region not previously studied, the present work is inteded to confirm the usefulness of the functions referred to above as well as to test the performance of the equation of state.Experimental The experimental device is based on an expansion principle.2 Both method and apparatus have been described previously in the literat~re.~.~ Temperatures were measured with an accuracy of 0.01 K, by using a Leeds and Northup calibrated platinum resistance thermometer, and were referred to the international tem- perature scale ITS-90. Temperatures in both the high-pressure cell and the expansion cell baths were controlled electronically to within kO.01 K. High pressures were mea- sured with a Heise bourdon gauge with an absolute accuracy of 0.01 MPa together with a Sensotec TJE1108-20 transducer with an accuracy of ca.0.02%. Both devices were calibrated against a Desgranges et Huot 5403 dead-weight gauge. The low pressures reached in the expansion system were mea- sured with a Maywood P-102 transducer with an accuracy of 0.07Yo. A reference density for m-xylene, p (298.15 K, 0.1 MPa) = 8.1011 mol dm-3 was taken from ref. 5. The second virial coefficient at 333.15 K of m-xylene in the vapour phase was calculated from data of Cox and Andon.6 Although the apparatus is essentially the same as that used for mesitylene,' a calibration procedure was necessary due to a rearrangement of the expansion system. Since accurate measurements have been reported in the literature for toluene by many this substance was chosen to calibrate our experimental device.Three isotherms, 248.15, 273.15 and 298.15 K, were mea- sured and compared to direct measurements reported by .~Mopsik7 and Kashiwagi et ~1 at the same temperatures. Absolute differences in density were always less than 0.004 mol dm-3 with the data of Mopsik and 0.0025 with those of Kashiwagi. The second virial coefficient at 333.15 K of toluene in the vapour phase was calculated from data of Scott et a/.'' Two additional isotherms, those of 223.16 and 303.14 K, were measured in order to check the quality and consistency of the calibration procedure. The 48 experimental ppT points are recorded in Table 1. All of the data are plotted in Fig. 1. Differences in our density results along the 223 K isotherm compared with those reported by Mopsik7 and Muringer et aL9 at the same temperature are ca.0.002 mol dm-3 on average if the data are referred to the same density at 0.1 MPa. Toluene, with a purity greater than 99.5%, was sup- plied by Carlo Erba. The accuracy of the densities reported here for both toluene and rn-xylene is always greater than 0.003 mol dm-3. The uncertainty in xT is ca. 0.02-0.03 GPa-'. a, for m-xylene is accurate within 0.03 kK-' at high pressures and within 0.02 K-at effectively zero pressure. m-Xylene, with a purity greater than 99%, was supplied by Carlo Erba. 10.6 10.4 10.2 I E 0 10.0---.9.8P 9.6 9.4 I' 0 10 20 30 40 50 60 70 80 90 100 110 120 PIM Pa Fig. 1 Comparison of densities of toluene. (0)This work, Table 1 ; (0)this work, isotherms measured for calibration ;(0)ref.7; (0)ref. 8; (A) ref. 9. T = (a) 223.16, (b) 248.15, (c) 273.15, (d) 298.15 and (e) 303.14 K. Table 1 Experimental values of density, plmol dm-3, of liquid toluene for pressures, plMPa, and temperatures, T/K P P P P P P P P T = 223.16 102.82 10.606 74.0 1 10.482 46.63 10.362 16.78 10.218 99.31 10.591 70.56 10.467 43.40 10.347 14.05 10.204 95.68 10.575 67.05 10.452 40.25 10.333 11.37 10.190 92.01 10.560 63.58 10.437 37.14 10.319 8.67 10.175 88.40 10.545 60.15 10.422 33.99 10.304 6.02 10.160 84.81 10.529 56.72 10.407 30.9 1 10.289 3.43 10.145 81.22 10.513 53.34 10.392 27.49 10.273 1.08 10.132 77.65 10.498 50.00 10.378 19.59 10.233 0.13 10.126 T = 303.14 105.50 9.987 62.12 9.756 36.52 9.596 14.32 9.434 99.10 9.954 55.36 9.716 30.65 9.556 9.29 9.393 91.22 9.913 48.86 9.676 25.02 9.5 15 4.47 9.351 69.05 9.796 42.57 9.636 19.57 9.475 0.39 9.314 Results and Discussion Freezing Pressures The melting curve of m-xylene was measured in the range 8-101 MPa following the procedure described in ref.1 and 11. The results, taken along two different series, are recorded in Table 2. Coexistence pressures are accurate to within 0.6 MPa. The melting point obtained by extrapolation of the measurements at 0.1 MPa is (224.9 0.3) K in good agree- ment with values found in the literature: 225.27 K,12 225.28 K.13 ppT Results and Mechanical Coefficients The 182 experimental ppT points, 30 along an isobar and 152 along eight isotherms, are recorded in Tables 3 and 4.These results have been fitted to an EOS (hereafter referred to as EOS1) recently proposed by our group to represent experi- mental ppT data of 1iq~ids.l~ This EOSl is an extension of the expression derived by Alba et a1.” The derivation of both EOS starts by representing the iso- therms of a, as a function of the pressure, p, by a simple power function’ a,@) = a*@ -pJ-1/2 (1) where ps is the divergence pressure along the so-called spino- dal curve16 and a* is a proportionality constant. This expression together with the widely known experi- mental observation of intersections occurring at high pres- sures for the isotherms of aP,l7-l9 led Alba and co-workers to derive a simple expression for representing the general Table 2 Experimental freezing pressures for m-xylene 249.43 101.2 250.43 105.3 247.76 94.1 248.97 100.1 246.68 89.6 247.66 94.7 245.34 83.9 245.66 86.6 243.34 77.1 243.70 78.2 242.75 73.0 241.77 69.7 241.55 68.0 239.76 61.4 240.10 61.4 237.13 50.5 238.94 56.7 234.92 41.4 237.69 51.6 232.99 33.2 235.91 44.4 230.83 24.9 233.98 36.5 229.44 19.1 232.31 29.9 227.42 11.2 230.38 22.2 226.52 7.9 228.40 14.3 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Experimental values of density, p/mol dm-3, of liquid m-xylene at temperatures, T/K,along the isobar of 0.37 MPa T P T P 226.15 8.704 263.69 8.397 229.80 8.676 266.37 8.375 231.73 8.661 268.96 8.353 234.07 8.641 27 1.27 8.334 236.51 8.622 273.73 8.314 238.69 8.602 276.35 8.292 241.37 8.580 278.93 8.272 243.46 8.562 281.75 8.248 245.88 8.543 284.33 8.227 248.83 8.519 286.94 8.205 251.20 8.499 289.28 8.180 253.59 8.480 291.84 8.157 256.32 8.457 294.57 8.133 258.84 8.440 296.65 8.115 261.41 8.419 299.23 8.092 behaviour of ap on the following basis: (a) the intersection of the ap isotherms occurs at a single point, (b) the spinodal originates at the critical point and (c) p,(T) can be repre-sented by a [1/1] Pade approximant which expresses ps as a function of a reduced temperature, t = (1 -T/T,),where T,is the critical temperature.The final expression for ap as a func- tion of t and the pressure, p, is: with p, given by Pm = pc(a1t + w2 t + 1) (3) where a1 and a2 are characteristic parameters for each sub- stance. The EOS can be directly obtained by integration of the standard thermodynamic relation In V = apdT. Since the complete derivation of the EOS is fully described in ref. 14, we shall give here a brief description of the more relevant remarks. Performing the integration between a reference tem-perature T,and the temperature T, the whole ppT surface of the liquid can be obtained from any reference isotherm in molar volume V (or molar density p). The final expression for the EOS is the following In [ = iz 7(a + b)F::: i: +{[-I -[-] F} (4) where R = (1 -o)aoT,/a,, o= (a1/a2),LI = (a-po/pc), b = (P/Pc -4and x = -[(Po -PJ(P -P31l’’.The function F depends only on x, a and b and takes dif- ferent forms for positive and negative values of the product (ab) and, if we call c = I ab Ill2, it can be written: For (ab) > 0 1F(a, b, x) = -[arctan(xc/a) -arctan(x, c/a)] (5)C For (ab) < 0 1 [“+ xc)(a -x.31F(a,b, x) = -In (6)2~ (a -XCXU + X,C) Although eqn. (4) is relatively complex and a non-linear numerical procedure to determine the characteristic param- eters is required, it depends only on four parameters, all of them with a clear physical meaning. In our opinion, these J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Experimental values of density, p/mol dm-', of liquid m-xylene at pressures, p/MPa, and temperatures, T/K P P P P P P P P T = 229.95 13.67 8.741 7.01 8.7 10 3.55 8.693 0.14 8.677 10.80 8.728 T = 233.19 27.12 8.768 17.35 8.727 7.14 8.683 0.15 8.649 22.13 8.747 12.87 8.707 2.50 8.661 T = 238.15 55.33 8.842 39.77 8.783 24.35 8.720 7.50 8.646 50.24 8.823 34.55 8.762 17.72 8.691 3.61 8.626 44.98 8.803 29.60 8.742 12.85 8.669 0.32 8.608 T = 248.11 94.08 8.913 67.70 8.818 41.52 8.7 15 15.95 8.604 88.61 8.894 62.59 8.799 36.47 8.695 11.00 8.579 83.39 8.875 57.35 8.779 3 1.05 8.673 5.48 8.552 77.79 8.855 51.88 8.757 25.95 8.649 0.20 8.525 72.83 8.837 46.91 8.737 21.07 8.628 T = 263.15 109.31 8.871 78.05 8.755 47.10 8.629 15.59 8.483 104.27 8.852 72.93 8.736 41.12 8.606 10.54 8.456 99.08 8.834 67.60 8.7 15 36.69 8.583 5.22 8.428 93.90 8.815 62.49 8.694 3 1.38 8.559 0.17 8.400 88.60 8.796 57.33 8.674 26.03 8.534 83.22 8.775 52.24 8.652 20.92 8.509 T = 273.15 109.16 8.803 78.18 8.683 46.44 8.551 15.79 8.402 104.26 8.784 73.26 8.663 41.60 8.529 10.50 8.374 98.90 8.764 67.60 8.640 36.43 8.505 5.22 8.345 92.98 8.742 62.31 8.619 31.22 8.480 0.16 8.316 88.01 8.723 56.82 8.596 25.66 8.454 83.38 8.703 51.71 8.574 20.84 8.428 T = 283.17 109.28 8.737 78.08 8.615 46.80 8.480 15.33 8.321 104.32 8.717 72.51 8.593 41.38 8.455 10.22 8.292 98.65 8.696 67.93 8.572 36.25 8.429 5.14 8.263 93.49 8.676 62.09 8.548 31.09 8.403 0.14 8.233 88.30 8.656 56.89 8.527 25.88 8.376 83.01 8.635 51.81 8.504 20.60 8.349 T = 298.15 109.63 8.653 74.78 8.5 11 47.96 8.386 21.36 8.243 105.91 8.639 72.25 8.500 45.56 8.374 18.89 8.228 101.92 8.623 70.06 8.489 43.16 8.362 16.45 8.214 98.18 8.609 67.56 8.478 40.73 8.349 14.00 8.199 94.50 8.593 65.09 8.467 38.24 8.336 11.56 8.183 88.90 8.571 62.76 8.456 35.84 8.324 9.25 8.168 86.22 8.560 60.17 8.444 33.38 8.311 6.82 8.153 83.59 8.549 57.72 8.432 30.97 8.298 4.34 8.136 81.80 8.542 55.27 8.421 28.54 8.284 1.97 8.119 79.50 8.532 52.79 8.409 26.22 8.271 0.40 8.108 77.21 8.522 50.36 8.397 23.79 8.257 features justify its extensive use instead of other polynomial EOS, depending on a large number of parameters lacking in physical meaning, described in the literature.l4 In order to be consistent with the spinodal concept it is convenient to represent the reference isotherm by the follow- ing function (7) where p, is the divergence density along the liquid branch of the spinodal, K* is a proportionality constant related to the isothermal compressibility through the K&) = K*(P -ps)-0'85 (8) In summary, if eqn. (7) is used to represent the reference isotherm, the number of parameters required to represent the equation of state of a given liquid by means of EOSl is, except for the critical parameters, six: ao, po, u1 and a, for a,@, T), and p, and K* for the reference isotherm, since p, at the reference temperature is given by eqn. (3). The character- istic parameters of EOSl for m-xylene, obtained with a weighted least-squares procedure, are recorded in Table 5.1219 Table 5 Coefficients of EOSl determined by fitting the experimen- tal densities recorded in Tables 3 and 4 a,/kK-' po/MPa a, u2 p,/MPa" TJK" 0.904 24.14 -23.40 -1.067 3.56 619.0 ~~ " Ref. 22. These results have been obtained by using the isotherm of 298.15 K as the reference with parameters recorded in Table 6. Eqn. (7) reproduces the experimental densities of the refer- ence isotherm within 0.001 mol dm-3. The average deviation between experimental densities and those calculated from EOSl is 0.002 mol dm-', in agreement with our estimated uncertainty. From parameters a, and u2 we have calculated the diver- gence pressures at the experimental temperatures and each isotherm has been fitted to eqn.(7). The parameters p, and K* are also recorded in Table 6. Experimental densities are always reproduced within 0.001 mol dm-3. An interesting issue that deserves analysis is the influence of the reference isotherm on the values of the characteristic parameters of EOS1. Our experience reveals that this influ- ence is only significant when, as in the present case, a rela- tively small range of temperature is considered. In general, while variations in po are typically about 10-20 MPa, the values of a. do not change too much.14 Nevertheless, the global performance of the EOS, i.e. standard deviation in the density, derived properties, divergence pressures, etc. remains almost the same.Eqn. (2) takes into account the occurrence of a crossover of the a, isotherms at the point (ao,po) where (aolplaT),= 0. This assumption, as in the case of mesitylene,' is compatible with the estimated uncertainties of our a, results and the rela- tively narrow range of temperatures covered by our measure- ment~.~~This observation is in agreement with the relatively large uncertainty achieved in po . For other substances a clear displacement of the intersec- tions of the a, isotherms can be observed.'* However, it usually requires a temperature interval of 200 K or greater and an accuracy on a, of CQ. l-2%.'83'9 In such a case, one can follow an analogous scheme to that used to derive eqn. (4) by using the expression suggested by Ter Minassian et al.l8 for representing their a, measurements of toluene.Selected values of a, for m-xylene calculated from eqn. (2) at round values of pressures and temperatures are recorded in Table 7, and indicate, through the following standard ther- modynamic relation, (aC,/aP)T = -V/P)c@: + (aap/aT),l (9) that a minimum in the isobaric heat capacity C, is expected at ca. 130 MPa in the range of temperatures covered in this Table 6 Coefficients of eqn, (7) for the isotherms recorded in Tables 1 and 4 m-xy lene 298.15" 4.8758 38.909 -88.62 283.17 4.9764 37.881 -98.87 273.15 4.9994 37.869 -106.44 263.15 5.0 129 38.003 -114.66 248.11 5.0817 37.455 -128.52 238.15 5.1208 37.178 -138.84 233.19 5.1322 37.134 -144.38 toluene 303.14 5.4968 40.840 -81.84 223.16 5.2458 45.237 -181.02 * Data represent the reference isotherm which must be included in eqn.(4) to fit the ppT surface of m-xylene. Table 7 Thermal expansion coefficient, a$K -of liquid m-xylene computed from eqn. (2) at round values of pressure, p, and tem- perature, T p/MPa 233.15 243.15 253.15 263.15 273.15 283.15 293.15 0.0 0.96 0.97 0.98 0.99 1.00 1.02 1.03 10.0 0.94 0.94 0.95 0.95 0.96 0.97 0.97 20.0 0.92 0.92 0.92 0.92 0.92 0.92 0.92 50.0 -0.86 0.85 0.84 0.83 0.82 0.81 100.0 --0.75 0.74 0.72 0.70 0.68 work. Unfortunately, we have not found experimental data of C,at high pressures for rn-xylene but, in any case, owing to the appearance of the solid phase, a minimum in C, should not be observed experimentally for temperatures below 260 K.Selected values of IC~ for rn-xylene calculated from eqn. (4) at round values of pressures and temperatures are recorded in Table 8. Fig. 2 shows the IC~results computed by finite differences from the densities recorded in Table 4 at the tem- peratures 298.15 and 265.13 K. IC~values calculated from eqn. (4) are represented by continuous lines in order to show the reliability of derived thermodynamic properties obtained by using EOS1. Our results at effectively zero pressure agree well with values found in the literature for this substance (see Table 9). IC~values at high pressures obtained from experimental den- sities found in the literature are compared in the next section.Comparison with Other Measurements at Higher Pressures Eqn. (l), (7) and (8) described above have been used recently to correlate experimental measurements of mesitylene.' We 1 & 0.7 c I t 0-0.41 1 ULI0.30L, 40 60 PIMPa Fig. 2 Isothermal compressibility of m-xylene. Symbols : calculated by finite differences of densities recorded in Table 4. Lines: calculated from eqn. (4). T = (0)263.15 and (0)298.15 K. Table 8 Isothermal compressibility, K,/GPa- ', of liquid m-xylene computed from EOSl at round values of pressure, pand tem-perature, T p/MPa 233.15 243.15 253.15 263.15 273.15 283.15 293.15 0.1 0.58 0.60 0.63 0.67 0.71 0.77 0.83 10.0 0.54 0.56 0.59 0.62 0.66 0.71 0.76 20.0 0.50 0.53 0.55 0.58 0.62 0.66 0.70 50.0 -0.44 0.47 0.49 0.51 0.54 0.57 100.0 --0.37 0.39 0.41 0.42 0.44 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 9 Isothermal compressibility of m-xylene at 0.1 MPa from different sources T/K ref. 283.15 293.15 298.15 this work 0.77 0.83 0.86 21 0.788 -0.869 23 -0.846 0.874 24 -0.852 0.889 also studied the possibility of using the functions for extrapo- lation with satisfactory results. Since experimental measure- ments at high pressures are available in the literature for the two compounds studied here, at temperatures covered by our measurements, we present here a similar study to that of mesitylene. .~~Sun et~2 reported values of the thermophysical proper- ties of toluene from speed of sound data up to 260 MPa.Isothermal compressibilities are compared in Table 10 with those calculated from eqn. (8) with the parameters recorded in Table 6. Our results for pressures greater than 100 MPa are extrapolated values. Both sets of data agree within their estimated uncertainties. Yokohama etd2'reported high-pressure density results for rn-xylene at 283.15 and 298.15 K. Table 11 records the comparison between our isothermal compressibilities, calcu- lated from eqn. (8) with the parameters recorded in Table 6 Table 10 Comparison of isothermal compressibilities obtained for toluene in this work with those given in ref. 20 T = 223.15 K T = 303.15 K p/MPa this work ref. 20 this work ref. 20 0.1 0.55 0.545 0.95 0.965 20.0 0.50 0.499 0.80 0.802 40.0 0.45 0.460 0.69 0.689 60.0 0.42 0.427 0.6 1 0.605 80.0 0.39 0.399 0.55 0.541 100.0 0.36 0.375 0.50 0.490 120.0 0.34 0.354 0.46 0.449 140.0 0.32 0.335 0.44 0.414 160.0 0.3 1 0.318 0.40 0.385 180.0 0.29 0.303 0.37 0.360 200.0 0.28 0.290 0.35 0.338 220.0 0.27 0.277 0.33 0.319 240.0 0.26 0.266 0.32 0.302 260.0 0.25 0.256 0.30 0.287 Values of this work for pressures greater than 100 MPa are extrapo- lated values calculated from eqn.(8) using the parameters recorded in Table 5. Table 11 Comparison of isothermal compressibilities obtained for m-xylene in this work with those given in ref. 21. T = 283.15 K T = 298.15 K p/MPa this work ref. 21 this work ref. 21 lG.0 0.7 1 0.728 0.78 0.798 50.0 0.54 0.562 0.59 0.604 100.0 0.42 0.442 0.45 0.469 125.0 0.38 0.400 0.4 1 0.423 150.0 0.35 0.366 0.37 0.385 175.0 0.32 0.338 0.34 0.355 200.0 0.30 0.314 0.32 0.329 Values of this work for pressures greater than 100 MPa are extrapo- lated values calculated from eqn.(8) using parameters recorded in Table 5. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1221 and extrapolated up to 200 MPa, and those derived from the results of Yokohama. As in the case of toluene, differences are within the estimated uncertainty of this property. Differences in molar densities are always less than 0.2%. 2 3 4 W. B. Streett and L. A. K. Staveley, J. Chem. Phys., 1971, 55, 2495. J. C. G. Calado, P. Clancy, A. Heintz and W. B. Streett, J. Chem. Eng. Data, 1982,27, 376.V. G. Baonza, J. Nunez and M. Caceres, J. Chem. Thermodyn., 1989,21, 231. Summary and Conclusion 5 6 J. L. Hales and R. Townsend, J. Chem. Thermodyn., 1972,4,763. J. D. Cox and R. J. L. Andon, Trans. Faraday SOC., 1958, 54, Accurate density measurements obtained with an expansion technique have been reported for rn-xylene from 230 to 298 K and pressures up to 110 MPa. Toluene was used to calibrate our experimental device and density measurements along the isotherms of 223 and 303 K have also been reported for this liquid. The experimental results have been correlated in terms of an EOS which has been used to evaluate the mechanical coef- ficients a, and icT of this substance as functions of pressure and temperature. The most important feature of this EOS is that it depends on six adjustable parameters only, all of them 7 8 9 10 11 12 13 1622.F. I. Mopsik, J. Chem. Phys., 1969,50,2559. H. Kashiwagi, T. Hashimoto, Y. Tanaka, H. Kubota and T. Makita, Znt. J. Thermophys., 1982,3,201. M. J. P. Muringer, N. J. Trappeniers and S. N. Biswas, Phys. Chem. Liq., 1985, 14, 273. D. W. Scott, G. B. Guthrie, J. F. Messerly, S. S. Todd, W. T. Berg, 1. A. Hossenlop and G. J. Waddington, J. Phys. Chem., 1962,66,911. V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1992, %, 1932. K. S. Pitzer and D. W. Scott, J. Am. Chem. SOC., 1943,65,803. R. De Goede, G. M. Van Rosmalen and G. Hakvoort, Thermo- with a clear physical meaning. The ability of simple power functions recently proposed to correlate high-pressure results of density, isothermal com-pressibility and thermal expansion coefficient has been tested.In addition, the extrapolation capabilities of these functions have been confirmed with experimental results at higher pres- 14 15 16 chim. Acta, 1989,156,299. V. G. Baonza, M. Caceres and J. Nuiiez, J. Phys. Chem., 1993, 97,10813. C. Alba, L. Ter Minassian, A. Denis and A. Soulard, J. Chem. Phys., 1985, 82, 384. V. P. Skripov, in Metastable Liquids, Wiley, New York, 1974, p. 226. sures found in the literature for the two compounds studied in this work. Since the complete equation of state (EOS1) is directly related to these power functions, their extrapolation capabilities can be directly transferred to the EOS. The results of this work together with those of ref. 1 17 18 19 20 V. G. Baonza, M. Ciceres and J. Nuiiez, J. Phys. Chem., 1993, 97,2002. L. Ter Minassian, K. Bouzar and C. Alba, J. Phys. Chem., 1988, 92, 487. Ph. Pruzan, J. Phys. Lett., 1984,45, L-273. T. F. Sun, S. A. R. C. Bominaar, C. A. ten Seldam and S. N. suggest the general application of EOSl to represent high- pressure thermophysical properties of normal liquids over wide ranges of temperature and pressure. 21 22 Biswas, Ber. Bunsenges. Phys. Chem., 1991,95,696. C. Yokohama, S. Moriya and S. Takakashi, Fluid Phase Equilib., 1990,60,295. K. H. Simmrock, R. Janowsky and A. Ohnsorge, in Critical Data This work was supported by CICYT (M.E.C., Spain), Project NO.: PB92-0553. 23 for Pure Substances, Chemistry Data Series 11, DECHEMA, Frankfurt, 1986. D. Tyrer, J. Chem. SOC., 1914,105,2534. 24 A. J. Richard and P. B. Fleming, J. Chem. Thermodyn., 1981,13, References 863. 1 V. G. Baonza, M. Caceres and J. Nuiiez, J. Chem. SOC., Faraday Trans., 1994,90, 553. Paper 4/001256; Received 10th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001217

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1223-1225 Heats of Transport of Aqueous Tetraalkylammonium Hydroxides and the Electrophoretic Effect Derek G. Leaist* and Ling Hao Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 587 A conductivity cell has been used to measure the Soret coefficients of dilute aqueous tetraalkylammonium hydroxides (R,NOH; R = methyl, ethyl, propyl, butyl) at 25°C. The heats of transport calculated from the Soret coefficients decrease as the concentration is increased. In contrast to this 'normal' behaviour, the heats of transport of dilute aqueous Bu,NCI, Bu,NBr and Bu,NI decrease and then increase as the concentration is raised. Owing to the large differences in the radii and heats of transport of the NBu,+ and halide ions, an abnormally strong electrophoretic effect is predicted for the tetrabutylammonium halides.The nearly identical heats of transport of the NBu,+ and OH- lead to a weak electrophoretic effect for aqueous Bu,NOH, which may account for the normal concentration dependence of its heat of transport. When a temperature gradient is imposed on a convection-free solution, some of the components diffuse to the warmer regions of the solution while others diffuse to the cooler regions.'-3 Eventually a steady state is reached in which the diffusion driven by the temperature gradient is balanced by ordinary diffusion down the thermally induced concentration gradients. The Soret coefficient 0 is a measure of the maximum thermal separation that can be achieved.It gives the fractional change in solute molality per degree under steady-state conditions g= -A(*) dT steady state The Soret coefficients of many solutions are in the range 0.001 to 0.01 K-', though a few remarkable systems such as CC14-CH30H4 and sodium dodecylsulfate-water' have Soret Coefficients as large as 0.03 K-'. Thus a 10 K tem-perature difference can, in favourable cases, produce a 30% change in composition. Large Soret coefficients (up to 0.016 K-') have also been reported for aqueous tetraalkylammonium salts.6 Tetra- alkylammonium ions are strong structure makers as a result of hydrophobic hydration. When a tetraalkylammonium ion diffuses in water, the solvent molecules left behind become more disordered.The heat absorbed to maintain the tem- perature in the ion's wake makes a large positive contribu- tion to the heat of transport, Q*. The heat of transport and Soret coefficient of a 1 : 1 electrolyte are related by Q* = 2oRT2(1 +-z:) where y* is the mean ionic activity coefficient and rn is the electrolyte molality. Electrostatic interactions cause the heat of transport of dilute electrolytes to drop sharply as the concentration is increased. In a recent thermocell study,6 however, the heat of transport of tetrabutylammonium chloride was found to pass through a minimum near 0.002 mol kg-', and then steadily increase. In a subsequent conductimetric study,7 retrograde heats of transport were also reported for aqueous tetra-butylammonium iodide and bromide.Though a convincing explanation for the unusual thermal diffusion behaviour of the tetrabutylammonium halides has not been given, the electrophoretic effect has been impli- cated.8 The limiting law for the heat of transport of an elec- trolyte can be written as2v3 Q*(m) = Q*' + Q*'Jm (3) where Q*' is the molar heat of transport at infinite dilution and Q*' is the limiting slope of Q* uersus the square root of the concentration. The limiting slope is the sum of the contri- bution QZ' from direct ion-ion electrostatic interactions and the electrophoretic contribution Q,*'from ion-solvent inter- actions similar to those in isothermal conductance and diffusion' Q*' = QZ' + Q,*' (4) The electrostatic term QZ' for a 1 : 1 aqueous electrolyte is -10.7 kJ kg'/' mol-3/2 at 25 "C." The electrophoretic term is much more difficult to e~aluate.~*~~"*'~According to Agar's hydrodynamic the~ry,~.~ $,*' is approximately pro- portional to the product of the differences in the Stokes' law radii and heats of transport of the ions.For a 1 :1 aqueous electrolyte at 25 "C the predicted electrophoretic term is Q,*' = 0.164 x 10" x (rs+-r,-)(Q*+O-Q?') kg'/' mol-'/2 m-' (5) where rs+ , I,-and Q;', Q?' are the Stokes' law radii and limiting heats of transport of the cation and anion. For the majority of electrolytes the differences in the ionic radii heats of transport are rather small. Consequently, the electrophoretic effect in thermal diffusion is usually negligible.For aqueous NaCl at 25 "C (rs+-rs-x 0.63 x lo-'' m, Q:' -Q?' x 2.9 kJ mol-'),9*13 the electrophoretic slope amounts to only 3% of the electrostatic slope. The situation is quite different for the aqueous tetra-butylammonium halides. For these salts the differences in ionic radii and heats of transport are both relatively large:'^'^ rs+ -rs-z 4.1 x lo-'' m, Q?' -Q?' x 20 kJ mol-'. This leads to an abnormally strong electrophoretic effect. In fact, the electrophoretic term is so large that it exceeds the magnitude of the electrostatic term and the heats of transport are predicted to increase with Concentration. Unfortunately, the suggestion that electrophoresis is responsible for the retrograde heats of transport of the tetra- butylammonium halides cannot be tested reliably until an accurate expression is developed for Q,*'(and perhaps higher- order terms).In the meantime, however, a qualitative test could be made by measuring the heat of transport of aqueous tetrabutylammonium hydroxide. Because the heats of trans- port of NBu,' and OH-are both large (20.79 and 17.2 kJ mol-', re~pectively)'~the difference QT0 -Q*-Ois relatively small. According to Agar's the~ry,~,~ this should lead to a weak electrophoretic effect and hence a 'normal' concentra- tion dependence is predicted for the heat of transport of aqueous Bu,NOH. On the other hand, if the unusual concen- tration dependence is caused by a peculiar property of the aqueous NBu, ion alone (such as hydrophobic hydration), + then all tetrabutylammonium electrolytes, including Bu,NOH, should have retrograde heats of transport.These considerations prompted us to measure the heats of transport of dilute aqueous Bu,NOH solutions. Results for aqueous Me,NOH, Et,NOH and Pr,NOH solutions are also reported. Experimental Carbonate-free solutions of Me,NOH, Et,NOH, Pr,NOH or Bu,NOH were prepared by passing aqueous solutions of tetraalkylammonium bromides or iodides (Aldrich or Fluka, >99% purity) through a column of anion-exchange beads (Amberlite 400) that had been treated with 1 mol dm-3 aqueous NaOH. The hydroxide solutions were analysed by potentiometric titration against standardised aqueous HCl and then diluted by mass to the required molality.Soret coefficients were measured with an Agar-Turner type A conductivity cell.14 The solution chamber was a 0.6 cm diameter hole in the centre of a 5 cm diameter, 1.21, cm thick Lucite disc.' A steady temperature gradient was maintained by clamping the cell between upper and lower nickel-plated copper cylinders held at 30.0 and 20.0°C, respectively. The changes in concentration caused by thermal diffusion were followed by measuring the resistances across pairs of 0.25 mm diameter platinum-wire electrodes at 1/6 and 5/6 of the cell height. Cell resistances were measured with a GenRad model 1689 self-balancing bridge operated at 1 kHz and 0.5 V. The effects of small drifts in the cell resistances were minimised by working with the differential quantity J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 where RJf) and RL(t) are the resistances across the upper(U) and lower(L) electrode pairs at time t. The relaxation time for thermal diffusion in a cell of height h is z= h2/n2D,where D is the mutual diffusion coefficient evaluated at the mean cell temperature (25.0 "C). For times t > 2/6, Y(t)decays exponentially to the steady-state value Y, Y(t) = Y, + Yl exp(-t/z) (7) The method of linear least squares was used to fit eqn. (7) to plots of Y(t)us. exp(-t/z). Soret coefficients were calcu- lated from values of the initial slope Yl according to the relation l6 B = -n2Yl/2J(3)BA T (8) The factor B = -(a In R/a In m)T was evaluated from plots of the log of the mean cell resistance us.the log of the mean cell molality. Check runs made with 0.0100 mol kg- ' KCl or &SO, gave Soret coefficients within +_0.0002K-' of the values obtained by previously by Snowdon and Turner17 with a type B (centre-tap) conductivity cell. The mutual diffusion coeffcients of the hydroxides were measured at 25°C by the Taylor dispersion method.18 Samples of solution were injected into laminar carrier streams at the entrance to a 3658 cm long capillary tube (0.04338 cm inner diameter). A differential refractometer detected the eluted peaks. Values of D were calculated by fitting the dispersion equation to the measured refractive index profiles. Results and Discussion Soret Coefficients The Soret coefficients of the tetraalkylammonium hydroxides were measured at the mean cell temperature of 25.0 +_ 0.1 "C and at molalities from 0.002 to 0.040 mol kg-'.The results are summarised in Table 1 together with the supplementary values of D and B that were used in the analysis of the resist- ance data. For the Me,NOH, Et,NOH and Pr,NOH solutions, the plots of Y(t)against exp( -t/z) were linear and the calculated Soret coefficients were reproducible within & 0.0002 K- '. During the later stages of the runs made with NBu,OH, however, convective remixing caused the slope of Y(t) us. Table 1 Soret coefficients and heats of transport of aqueous tetraalkylammonium hydroxides at 25 "C R,NOH m/mol kg- 11/10-~ cmz s-l a/K-' B 1 + d In y,/d In m Q*/kJ mol-' Me,NOH 0.002 1.94 0.0182 0.999 0.975 26.2 0.005 1.92 0.0176 0.993 0.964 25.1 0.010 1.89 0.0166 0.985 0.952 23.4 0.020 1.87 0.0161 0.976 0.937 22.2 0.030 1.86 0.0138 0.97 1 0.927 18.9 Et,NOH 0.002 1.47 0.02 13 0.997 0.975 30.7 0.005 1.46 0.02 14 0.983 0.962 30.4 0.010 1.44 0.02 13 0.972 0.949 29.9 0.020 1.41 0.0202 0.962 0.932 27.8 0.030 1.39 0.0195 0.956 0.920 26.5 Pr,NOH 0.002 1.10 0.0238 0.992 0.975 34.2 0.005 1.08 0.0243 0.976 0.962 34.6 0.010 1.04 0.0241 0.964 0.947 33.7 0.020 1.03 0.0235 0.954 0.929 32.3 0.030 1.02 0.0226 0.945 0.9 17 30.6 Bu,NOH 0.002 0.94 0.0261 0.985 0.975 37.6 0.005 0.93 0.0259 0.968 0.962 36.8 0.010 0.92 0.0267 0.955 0.948 37.4 0.015 0.91 0.0265 0.947 0.938 36.7 0.020 0.9 1 0.0264 0.942 0.930 36.3 0.030 0.89 0.0262 0.934 0.918 35.6 0.040 0.87 0.0265 0.929 0.910 35.6 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 exp(-tt/z) to decrease gradually. As the length of the alkyl chain increases the density derivative dpldrn decreases, so aqueous Bu,NOH solutions are least stable against convec- tion. Nevertheless, Soret coefficients reproducible within &O.O004 K-' could be determined for aqueous Bu,NOH solutions from the linear initial portion of the Y(t)us. exp(-t/ 7)plots. Heats of Transport Activity-coefficient data are required to calculate heats of transport from Soret coefficients. Such data do not appear to have been reported for aqueous tetrabutylammonium hydroxides.Fortunately, however, the solutions of interest to the present study are sufficiently dilute that it is acceptable to evaluate the activity coefficients from Debye-Huckel theory or related semi-empirical equations. We used values of y* calculated from Pitzer's equation for the appropriate tetra- alkylammonium chloride. '9*20 The heats of transport of the tetraalkylammonium hydrox- ides are listed in Table 1 and plotted against the square root of the molality. For each hydroxide, including Bu,NOH, the values of Q* decrease steadily as the molality is raised. Fig. 1 includes for comparison the heats of transport of the tetra- alkylammonium chlorides determined by Lin and co-workers6 in a recent thermocell study.The intercepts plotted in Fig. 1 are the limiting heats of transport calculated from limiting single-ion heats of trans- port listed in Table 2. The latter were estimated by using Takeyama and Nakashima's reduction rule' which is based on Q*O(Cl-) = 0.53 kJ mol-'. The measured heats of trans- port of the tetraalkylammonium hydroxides appear to extrapolate to values which are acceptably close to the pre- dicted Q*O values. This check suggests that the present mea- surements are free of gross systematic errors from convection. 1225 Table 2 Predicted limiting slopes of Q* us. rn'" for aqueous tetra- alkylammonium electrolytes at 25 "C R,NOH I,+r,-Q*+O Q*-O QZ' Q:' Q*' Me,NOH 2.05 0.46 10.0 17.2 -10.7 -1.9 -12.6 Et,NOH 2.82 0.46 14.3 17.2 -10.7 -1.1 -11.8 Pr,NOH 3.93 0.46 18.4 17.2 -10.7 0.7 -10.4 Bu,NOH 4.74 0.46 20.8 17.2 -10.7 2.5 -8.2 Me,NCl 2.05 1.21 10.0 0.53 -10.7 1.3 -9.4 Et,NCl 2.82 1.21 14.3 0.53 -10.7 3.6 -7.0 Pr,NCI 3.93 1.21 18.4 0.53 -10.7 8.0 -2.7 Bu,NCl 4.74 1.21 20.8 0.53 -10.7 11.8 1.1 The dashed lines shown in Fig.1 are the limiting slopes of Q* us. J(m) calculated according to eqn. (4) and (5). For aqueous Bu,NOH, the predicted slope is -8.2 kJ kg'/' m~l-~''. This value agrees reasonably well with the value -13 k5 kJ kg'" m~l-~''obtained by least-squares analysis of the Bu,NOH data shown in Fig. 1. Reasonable agreement is also obtained for Et,NOH and Pr,NOH below 0.01 mol kg-'.For Me,NOH, however, the predicted slope appears to be too shallow. In conclusion, a weak electrophoretic effect is predicted for the thermal diffusion of dilute aqueous Bu,NOH, and the limiting concentration dependence of its heat of transport is adequately represented by existing theory. This supports the suggestion that the large, positive electrophoretic term pre- dicted for the tetrabutylammonium halides accounts for the unusual concentration dependence of their heats of transport. With an improved expression for the electrophoretic terms in thermal diffusion it might be possible to verify the last of the limiting laws for electrolyte solutions. The authors thank the Natural Sciences and Engineering Research Council of Canada for the financial support of this research.References 40 21 H. J. V. Tyrrell, Diffusion and Heat Flow in Liquids, Butter-0.0 0.1 0.2 C.3 m'i2/mo(lI2kg312 Fig. 1 Heats of transport of dilute aqueous tetraalkylammonium hydroxides (this work) and chlorides (ref. 6) plotted against the square root of the molality. (---) Predicted limiting slope [eqn. (4)].(O),(+) Limiting heats of transport predicted by Takeyama and Nakashima's reduction rule.' (0)Bu,NOH ; (V)Pr,NOH ; (H)Et,NOH; (A) Me,NOH; (0) Et,NCl;Bu,NCl; (V) Pr,NCl; (0) (A) Me,NCl. worths, London, 1961. 2 J. N. Agar, Advances in Electrochemistry and Electrochemical Engineering, 1963, vol. 3, p. 31. 3 J. N. Agar, in The Structure of Electrolytic Solutions, ed.W. J. Hamer, New York, 1959, ch. 13. 4 M. J. Story and C. J. R. Turner, Trans. Faraday SOC., 1969, 65, 1523. 5 D. G. Leaist and €3. Lu, J. Phys. Chem., 1989,93,7547. 6 C. J. Petit, M. Hwang and J. Lin, J. Solution Chem., 1987, 17, 1. 7 H. Lu and D. G. Leaist, J. Solution Chem., 1991, 20, 199. 8 D. G. Leaist, J. Phys. Chem., 1991, M,7134. 9 R. A. Robinson and R. H. Stokes, Electrolyte Solutions, Aca-demic Press, New York, 1959,2nd edn., appendix 6.1. 10 P. A. Wilmarth and A. D. Payton, .I.Phys. Chem., 1981, 85, 3590. 11 E. Helfand and J. G. Kirkwood, J. Chem. Phys., 1960,32, 857. 12 T. S. Thacher, C. Y. Mou, U. Mohanty and J. Lin, J. Chem. Phys., 1986,84,6401. 13 N. Takeyama and K. Nakashima, J. Solution Chem., 1988, 17, 305. 14 J. N. Agar and J. C. R.Turner, J. Phys. Chem., 1960,64,1000. 15 D. G. Leaist, J. Solution Chem., 1989, 18, 651. 16 D. G. Leaist and L. Hu, J. Phys. Chem., 1990,94,447. 17 P. N. Snowdon and J. C. R. Turner, Trans. Faraday SOC., 1960, 56,1812. 18 D. G. Leaist, J. Chem. SOC., Faraday Trans., 1991,87, 597. 19 K. S. Pitzer and G. Mayorga, J. Phys. Chem., 1973,77,2300. 20 K. S. Pitzer, J. Phys. Chem., 1974,78,2698. Paper 4/002051; Received 13th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001223

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1227-1231 Complexation and Precipitation Equilibria in the System Ni"-Cr"'-H,O R. Castaiio," M. A. Olazabal, G. Borge and J. M. Madariaga Kimika Analitikoaren Departamentua, Euskal Herriko Unibertsitatea, P.K. 644,E-48080 Bilbao, Spain A potentiometric and spectrophotometric study of the complexation equilibria between Cr"" and Nil has been performed. Formation of the soluble NiCrO, complex has been found and the thermodynamic formation constant has been calculated (log b';, = 2.40 f0.03), as well as its molar absorptivity values at the different wavelengths studied. The precipitation equilibria in the system Ni"-CrV'-H20 have also been investigated. A mixed precipi- tate, NiCrO, * 3Ni(OH), , has been found and its thermodynamic solubility constant has been determined (log K io= -51.1 & 0.2).Hydrometallurgical processes produce industrial wastes con- taining moderate or high metal concentrations, some of which have a high toxicity, making necessary their elimi- nation before waste disposal. In some cases, the elimination or recovery of the metals is hindered because of the lack of information about the equilibria taking place between the dif- ferent components present. One of the processes that produces waste waters with high toxicity is that corresponding to chromium plating, owing to the high toxicity of Cr". In these kinds of waste waters some transition metals like Fe'", Ni" and Cu", and Ba" are present. There is enough information about the CrV'-Ba" interaction in the literature'.' to conclude the formation of the BaCrO,(s) precipitate.In the case of Feu' and Cu" there is very little information in the literature, except for the prob- able presence of the soluble complex FeCrO,' in the Fe"' system3v4 and the CuCrO,(s) precipitate in the Cu" system.' However, no information has been found on the equilibria in the Nin-Crv' system. This lack of knowledge makes necessary an experimental study of this system. Taking into consideration the similar behaviour of the Cr0,' -anion and other inorganic anions like SO,' -, HPO,'-and C0,2- and the available information about the formation of soluble complexes and precipitates between these anions and Ni2+ (SO,'-gives a soluble complex with Ni", NiSO,;' HPO,'-a neutral complex' and CO,'- forms basic non-soluble carbonates, which are mixed precipi- tates with carbonate and metal hydroxideg), the formation of soluble and mixed solid species in the Ni1'-CrV1-H2O could also be expected.The present work was designed in order to elucidate the complexation and precipitation equilibria in the Nin-CrV'-H 20system. Knowledge of the thermodynamic equilibrium constants allows further information on the system to be obtained for different ionic media provided that an adequate method for activity coefficient estimation is available. Instead of this, information on the stoichiometric stability constants is valid only for the specific medium studied. The problem with the activity coefficients can be solved if the ionic strength used for the study of the equilibria is low, because estimation of the activity coefficients can then be performed in a very simple way.In this work the experimental conditions selected are such that data treatment can be carried out directly on the molar activity scale in order to obtain the thermodynamic stability constants of the system. Experimental Reagents The chemicals used were all of analytical grade. Stock solu- tions were prepared: K2Cr04 (Merck, pa) solutions at 5.0 x and 1.0 rnol dm-, concentrations. The Ni(NO,), solutions at 9.51 x and 0.1 mol dm-, were stan-dardized through a complexometric titration against EDTA as titrant and murexide as indicator." Solutions of HNO, (Fluka, pa) at 1.0mol dm-, and KOH (Merck, pa) at 0.1 and 1.0 mol dm -were standardized volumetrically against tris(hydroxyrnethy1)aminomethane" and potassium phthalate" using methyl red and phenolphthalein as indica-tors, respectively.Experimental Technique Two kinds of experiments were performed in this work corre- sponding to the complexation study and to the precipitation equilibria study. Complexation Equilibrium A potentiometric-spectrophotometric study of different solu- tions containing Ni" and Ni"-Crv' mixtures was carried out at different pH and metal concentration levels. A conventional pH cell: Ag I AgCl(s)IKC1 (3 mol dm- ') !test solution I glass electrode (0 was used to measure the activity of H+ in solution. The test solution had a variable composition of A mol dm-3 Ni(NO,), + B mol dm-, K,CrO, + C, mol dm-, HNO, or A mol dm-, Ni(N0,)' + B mol dm-, K,CrO, + C2 mol dm-3 KOH without constant ionic strength, but this was always known and <0.1mol drn-,.The Ni" concentrations studied were 1.0 x lop3, 5.0 x lop3 and 1.0 x lod2 mol drn-,, and for each Ni" concentration another three levels of CrV' concentrations (2.40 x lo-,, 3.68 x lo-, and 5.06 x lo-, mol dm-,) were studied. According to Bates,I2 and considering that the electrodic system was calibrated using buffer solutions of known activ- ity, the activity of H+ was calculated as {H+} = lopPH. The spectrophotometric measurements were carried out using a Hewlett-Packard 8452A Diode Array spectropho- tometer.In all cases the whole spectrum in the range 240-700 nm was recorded at 2 nm intervals. Prior to the study of the Ni"-CrV'-H20 system, the Ni"-H,O system was studied under the same experimental conditions. It was found that the spectrum did not vary with pH in this concentration range, which is in agreement with the absence of significant hydroxy complex formation in the pH range studied. Precipitation Equilibrium In order to minimize the formation of nickel hydroxide pre- cipitates, the test solutions were prepared in the following order: K,CrO, ,Ni(NO,), ,KOH. Table 1 Proposed results of the thermodynamic formation con-stant and molar absorptivities for the NiCrO, complex ~ results log fi;lo &274nm '350 nm '400 nm -eqn.(I) 2.409 k 0.035 3367 f 14 2558 9 1796 k 5 eqn. (11) 2.386 L-0.031 3460 k 19 2601 & 13 1818 11 proposed 2.40 i-0.04 3399 f 48 2573 & 33 1796 13 Reagent addition was performed dropwise under contin- uous stirring to avoid, as much as possible, local precipitate formation. The possibility of contamination and evaporation was minimized. Three sets of experiments were carried out covering the Nil'. CrV1 ratios: 2 : 1, 1 : 1 and 1 : 2. The test solutions in contact with the solid phase were kept at 25 1"C during the time needed to reach equilibrium, i.e. ca. one week. After that, the pH values of the saturated solu- tions, in contact with the precipitates, were measured with the galvanic cell described above [cell (i)].Analysis of nickel in solution was performed by atomic absorption spectros- copy (AAS) measurements using a Perkin-Elmer 560 spectro- photometer. On the other hand, analysis of chromium was carried out by redox titrations using Fe(NH,),(SO,) 6H,O as titrant and ferroin as indicator because there were inter- ferences of Ni" in the analysis by AAS. Practically all of the experiments revealed the presence of two kinds of precipitate, one blue (of nickel hydroxide appearance) and a second brown. Therefore, the solubility data might obey the saturation conditions for these two pos- sible precipitates. Results Complexation Equilibria The complexation equilibria in the Ni"-CrV1-H,O system can be explained by the following general equilibrium: pNi2+ + qOH-+ rCr042-Bwr (Ni),(OH)q(Cr04)~P-4-(1)Ir Owing to the variation of ionic strength in the pH range studied, graphical treatment of the data cannot be performed using the concentration scale.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The (-log{H+), AA1,A,, , . . . , data at different wavelengths and total concentrations of Ni" and CrV' were treated with the SPECA program,13 which takes into account the variation of the activity coefficient at each data point. Since the ionic strength is always <0.1 mol dm-3, calcu- lation of activity coefficients can be performed using the Debye-Huckel extended law. l4 A further analysis of system-atic errors was performed. SPECA minimizes the difference between the experimental absorbance and the absorbance calculated at each wave-length, assuming a set of species and their thermodynamic equilibrium constants. An iterative procedure must be solved for each data point because the absorbance equation uses equilibrium molar concentrations, while species formation is defined in terms of thermodynamic equilibrium constants.The program calculates a set of stoichiometric stability constants and a value of the ionic strength from the initial values of the thermodynamic constants and the total concen- tration of all of the species. With this set of constants and solving the mass balance equation it is possible to obtain the detailed composition of the system (i.e. the free concentration of each species). Once the system composition is obtained, it is possible to recalculate the ionic strength, I, of each solu- tion.With the value of the ionic strength and making use of the Debye-Huckel extended law the activity coefficients can now be calculated. Then, with this set of activity coefficients, it is possible to recalculate a new set of stoichiometric con- stants and restart the cycle of I and y calculation. This process is continued until the value of I and y converge. Then it is possible to obtain the calculated value of the absorbance for each point because the free concentration, of the species are known and the absorptivities, E, are given in the first instance. The program then varies the values of the ther- modynamic constants and E, until a minimum of the error- square sum is found.Two minimization strategies were employed: in the first, the sum of the squared deviations, NP and in the second, the sum of the relative squared deviations, were used for all the experimental points, N, . For each of the three selected wavelengths, 274, 350 and 400 nm, the (log{H+), A), data as well as the total concentra- tions of all of the non-reacting ionic species (K' and NO,-) are required by the program for ,each experimental point in order to calculate the ionic strength and perform the correc- tions for activity variations. In all cases, the absorbance due to the Ni contribution was subtracted from the values corre- sponding to the Ni"-Crv1 system in order to simplify the data treatment. The different statistical parameters, U, 0 (standard deviation) and R (Hamilton factor) obtained for several models with different (p, q, r) stoichiometric values, employed for the two minimization strategies, (J= -3 4 5 6 7 (N, N,)'120.3i -log{ H+} Fig.1 Experimental data, A =f( -log(H+}), for the Ni"-Cr"'-H,O system at I = 350 nm, for CCrvI= 2.40 x mol i= 1 dm-3 and different CNil1:(0)1.0 x (A) 5.0 x and (0)1.0 x mol dm-3 where N, is the number of computed constants. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Thermodynamic formation constants of the hydrolysis, complexation and precipitation equilibria used to ascertain the stoi-chiometry of the mixed precipitate between Nil' and CrV1 species Ni2+ H+ CrO,'-log /?" ref.HCr0,-0 1 1 6.58 16 Cr,0,2 -0 2 2 14.55 16 NiOH+ 1 -1 0 -9.86 15 Ni( OH), 1 -2 0 -19.0 15 Ni,(OH)3+ 2 -1 0 -10.7 15 Ni,(OH),*+ 4 -4 0 -27.74 15 NiCrO, 1 0 1 2.40 a OH-0 -1 0 -14 15 Ni(OH), 6) 1 -2 0 -12.4 19 -10t -,-12 hNI Q -14-!$ h ;--16-z z 0'-18 -%I \{1-201 1 I t7.0 7.5 8.0 8.5 -log{H +} Fig. 2 L0g((Ni~+)~{Cr0,'-))as a function of -log{H+} 7 -4 . I1 I I 6.0 6.5 7.0 7.5 8.0 8.5 -log{H +I Fig. 3 Solubility of Ni" as a function of pH for three different Nil' :Cr"' ratios: (0)2 : 1, (0)1 : 1, (H)1 :2 1229 I I I I I , -1.5 " e >,:-2.0- -0 0 -2.5 I . I - Precipitatioa Equilibrium The solids obtained for each data point were checked by X-ray diffraction and the absence of NO3- and K+ in the precipitates was confirmed.Therefore, the presence of only Ni", CrV1 and OH- (or H+)can be assumed. Even if the nickel hydroxide is present, the formation of new insoluble species from the Ni2+, H+and Cr042- com- ponents can be expressed by the following general equi- librium : pNi2+ -4H+ + e(Ni),(H)-,(CrO,),(s) (2) with the corresponding thermodynamic solubility product: Kb* = {Ni'-}J'(H'}-~{CrO,"}' and the electroneutrality condition expressed by : 2p -q -2r = 0 (Iv) Taking into account the variability of the ionic strength, it was decided to work with activities to define the stoichio- metry of the precipitates as well as the thermodynamic con- stants. The MASBAC program" was used to compute the activity of each species in solution making use of the experi- mental {H'} data, the total concentration of all components (Ni", Cr", K+, NO3-) in the saturated solutions and taking into consideration the chemical model shown in Table 2.With knowledge of the {Ni2+}, {H+} and {Cr042-} values for each data point, a graphical treatment can be performed to ascertain the stoichiometries of the precipitates. Several log({Ni2+}p{Cr0,2-}~ us. log{H+} models were tested in order to obtain a suitable straight line with a slope adequate to fulfil the requirement of the electroneutrality condition. Fig. 2 shows the simplest combination with integer numbers which gave a straight line, with a slope equal to -6 for = 1 and P = 4.Thus, the Proposed stoichiometric indexes are: p = 4, q = 6, r = 1. The log Kh* value was obtained from the origin intercept of the plot in Fig. 2: log Ki: = 32.9 f 0.2. If the formation of the solid is defined with OH- as a com- ponent, its stoichiometry could be written as NiCrO, * 3Ni(OH), and its thermodynamic solubility product would be expressed as: KL = {Ni2c}4{OH-}6{Cr042-) with a value of pK,", = 51.1 f0.2. Discussion The thermodynamic solubility product of the mixed precipi- tate together with the thermodynamic formation constants of the soluble species, collected in Table 2 have been used in MASBAC to plot the theoretical solubility curves of Nil' and CrV1 shown in Fig. 3 and 4. Fig. 5 shows the distribution of the Ni" species with pH.This plot has been constructed with the ISP software package,18 keeping the ionic strength fixed at 0.2 mol dm-3 and considering all the species of the system collected in Table 2. As can be seen, the two precipitates are nearly always present, which agrees with the experimental observa- tions. As is shown in Fig. 3, the solubility of Ni" decreases con- tinuously with increasing pH but with a varying slope, owing to the different mean composition of the solid phase. At pHs between 7.2 and 7.7, the mixed precipitate micro, 3Ni(OH),] is the major species in the solid phase, while NiCrO, is the major complex in solution (see Fig. 5). The variation of the solubility of Ni" is controlled by the ratio of Ni" and OH- in both major species.When the pH is higher than 7.7, Ni(OH),(s) becomes the major species in the solid phase, so the variation in the solubility changes accord- ing to the new ratio. In the case of Cr"' (Fig. 4), the solubility decreases only at the beginning of the precipitation, owing to the formation of the mixed precipitate. The solubility curve for Cr"' will then display a zero slope and subsequently a positive one, until the conditions of saturation of this mixed precipitate disappear. There is a very narrow pH range (<0.5 pH units) where the NiCrO * 3Ni(OH)2 precipitate is the most important Ni" species, as can be seen in the predominance area diagram for the Ni" species shown in Fig. 6, i.e. although this precipitate is present as a minor, but important, species at a higher pH range, as can be seen in the predominance area diagram for the CrV' species shown in Fig.7. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Ni2+ -log{H +} Fig. 6 Predominance diagram of the Ni" species as a function of pH and CCrvlfor constant CNiu= 2.0 x lop2mol dm-' The soluble NiCrO, complex is the most important for the Ni" species not only near saturation conditions but also at more acidic pHs, especially when the CrV' concentration increases. Although this species (NiCrO,) does not have a very high formation constant value, it is very important because of its predominance in the Ni" system. This is valid for the mixed precipitate as well, and both species are very important for modelling the Ni"-CrV'-H,O system and should be taken into account when new treatments for waste waters with these two components are designed.Finally, one of the most important contributions of this work is the proposed experimental methodology and data treatment for the calculations of thermodynamic equilibrium constants. These constants are usually calculated by means of different correlations from the experimental stoichiometric constants, obtained previously with a more classical experi- mental methodology. Knowledge of the thermodynamic equi- librium constants has a larger applicability, but they are difficult to determine experimentally because of the variation of the activity coefficients with ionic strength. As a result, there are no computer programs available for calculation of I-log{ H +} -3.04 6 a 10 12 14 Fig.5 Distribution diagram of Nil' species as a function of pH; -log{H +} CNiu= 4.0 x loA2mol dm-' and C,,,, = 5.6 x mol dm-'. (a) Fig. 7 Predominance diagram of the CrV1 species as a function of Ni2+,(b) NiCrO,, (c) Ni,CrO,(OH),(s), (d)Ni(CH),(s). pH and CNiufor constant CCrVl= 4.0 x mol dm-' J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 equilibrium constants with data at variable ionic strength. Therefore the graphical method presented has been devel- oped in order to treat the data for the precipitation equi- libria. In order to simplify the calculations of the activity coefficients, the ionic strength of the solutions was kept at values <0.1 mol dm-3.Financial support of the University of the Basque Country, through Project UPV/EHU 1713 1O-E142/90, is gratefully acknowledged. References G. L. Beyer, and W. Rieman, J. Am. Chem. SOC., 1943,65,971. 0.Lukkari, and H. Lukkari, Suomen Kem., 1972,45B, 6. M.J. Burkhart and R. C. Thompson, J. Am. Chem. SOC., 1972, 94,2999. J. H.Espenson and S. R. Helzer, Inorg. Chem., 1969,8, 1051. S. Peterson and 0. W. Cooper, Trans. Kentucky Acad. Sci., 1951,13, 146. N.N.Greenwood and A. Earnshaw, in Chemistry of the Ele- ments, Pergamon Press, Oxford, 1984. S. Katayama, Bull. Chem. SOC.Jpn., 1973,46, 106. 1231 8 C. M. Fry and J. E. Stuehr, J. Am. Chem. SOC., 1972,94,8898. 9 F. Burriel, F. Lucena, S. Arribas and J. Hernandez, in Quimica Analitica Cualitativa, Paraninfo, Madrid, 1983. 10 G. H. Jeffery, J. Bassett, J. Mendham and R. C. Denney, in Vogel’s Textbook of Quantitative Chemical Analysis, Wiley, New York, 1989. 11 R. G. Bates and H. B. Hetzer, Anal. Chem., 1961,33, 1285. 12 R.G.Bates, CRC Crit. Rev. Anal. Chem., 1981, 247. 13 R. Cazallas, M. J. Citores, N. Etxebarria, L. A. Fernandez and J. M. Madariaga, Talanta, 1993,submitted. 14 R. A. Robinson and R. H. Stokes, in Electrolyte Solutions, Butterworths, London, 1959. 15 C. F. Baes and R. E. Mesmer, in The Hydrolysis of Cations, Wiley, New York, 1976. 16 M. A. Olazabal, G. Borge, R.Castaiio, N. Etxebarria and J. M. Madariaga, J. Solution Chem., 1993,22,825. 17 J. M. Madariaga, in preparation. 18 I. Puigdomenech, TRITA-OOK-.JOZO,Dept. Inorg. Chem., The Royal Inst. Technol. (KTH), Stockholm, September, 1983. 19 A. Ringborm, in Complexation in Analytical Chemistry, Wiley-Interscience, New York, 1963. Paper 3/06309G; Received 22nd October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001227

出版商:RSC    

年代:1994

数据来源: RSC